[nlohmann/json.hpp] fix warning with gcc 4.8

[linev]

By chance internal data member is the same as "subtype" argument

Fixes warnings like:

In file included from /mnt/build/jenkins/night/LABEL/ROOT-centos7/SPEC/noimt/V/6-22/root/io/io/src/TBufferJSON.cxx:131:0:
/mnt/build/jenkins/night/LABEL/ROOT-centos7/SPEC/noimt/V/6-22/root/io/io/res/json.hpp: In constructor ‘nlohmann::byte_container_with_subtype<BinaryType>::byte_container_with_subtype(const container_type&, uint8_t)’:

/mnt/build/jenkins/night/LABEL/ROOT-centos7/SPEC/noimt/V/6-22/root/io/io/res/json.hpp:4452:9: warning: declaration of ‘subtype’ shadows a member of 'this' [-Wshadow]
         : container_type(b)
         ^
/mnt/build/jenkins/night/LABEL/ROOT-centos7/SPEC/noimt/V/6-22/root/io/io/res/json.hpp: In constructor ‘nlohmann::byte_container_with_subtype<BinaryType>::byte_container_with_subtype(nlohmann::byte_container_with_subtype<BinaryType>::container_type&&, uint8_t)’:

/mnt/build/jenkins/night/LABEL/ROOT-centos7/SPEC/noimt/V/6-22/root/io/io/res/json.hpp:4458:9: warning: declaration of ‘subtype’ shadows a member of 'this' [-Wshadow]
         : container_type(std::move(b))
         ^
/mnt/build/jenkins/night/LABEL/ROOT-centos7/SPEC/noimt/V/6-22/root/io/io/res/json.hpp: In member function ‘void nlohmann::byte_container_with_subtype<BinaryType>::set_subtype(uint8_t)’:

/mnt/build/jenkins/night/LABEL/ROOT-centos7/SPEC/noimt/V/6-22/root/io/io/res/json.hpp:4493:5: warning: declaration of ‘subtype’ shadows a member of 'this' [-Wshadow]
     {
     ^

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos8-multicore/default, ROOT-fedora30/cxx14, ROOT-fedora31/noimt, ROOT-ubuntu16/nortcxxmod, mac1014/python3, mac1015/cxx17, windows10/cxx14
How to customize builds

[phsft-bot]

Build failed on mac1015/cxx17.
Running on macphsft20.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2020-10-23T11:45:36.858Z] CMake Error at cmake/modules/SearchInstalledSoftware.cmake:1576 (message):
• [2020-10-23T11:45:36.858Z] CMake Error at /Users/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1051 (message):

[phsft-bot]

Build failed on ROOT-ubuntu16/nortcxxmod.
Running on sft-ubuntu-1604-1.cern.ch:/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.python.pythonizations.roottest_python_pythonizations_pythonizations
• projectroot.roottest.python.pythonizations.roottest_python_pythonizations_smartptr
• projectroot.roottest.python.stl.roottest_python_stl_stl

[phsft-bot]

Build failed on ROOT-debian10-i386/cxx14.
Running on pcepsft10.dyndns.cern.ch:/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.python.pythonizations.roottest_python_pythonizations_pythonizations
• projectroot.roottest.python.pythonizations.roottest_python_pythonizations_smartptr
• projectroot.roottest.python.stl.roottest_python_stl_stl

[linev]

jenkins producing many errors, but they all has nothing to do with PR

[phsft-bot]

Build failed on ROOT-fedora30/cxx14.
Running on root-fedora30-1.cern.ch:/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.python.pythonizations.roottest_python_pythonizations_pythonizations
• projectroot.roottest.python.pythonizations.roottest_python_pythonizations_smartptr
• projectroot.roottest.python.stl.roottest_python_stl_stl

[phsft-bot]

Build failed on mac1014/python3.
Running on macitois19.cern.ch:/ec/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.roottest.root.io.webfile.roottest_root_io_webfile_make