Fix compiler warning in FoundationUtils.cxx

[linev]

Fix warning, seen in all recent PRs

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac11/noimt, mac12arm/cxx20, windows10/cxx14
How to customize builds

[hahnjo]

There already is #13073, I will merge it shortly...

[linev]

I see - miss it completely

[phsft-bot]

Build failed on mac11/noimt.
Running on macphsft23.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2023-06-22T15:42:57.561Z] CMake Error at /Users/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1142 (message):

[phsft-bot]

Build failed on ROOT-performance-centos8-multicore/cxx17.
Running on olbdw-01.cern.ch:/data/sftnight/workspace/root-pullrequests-build
See console output.

Errors:

• [2023-06-22T16:51:28.263Z] CMake Error at /data/sftnight/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1142 (message):

[phsft-bot]

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

Errors:

• [2023-06-22T16:54:13.537Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1142 (message):

[phsft-bot]

Build failed on mac12arm/cxx20.
Running on macphsft26.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2023-06-22T17:30:47.714Z] CMake Error at /Users/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1142 (message):

[phsft-bot]

Build failed on ROOT-ubuntu2004/python3.
Running on root-ubuntu-2004-1.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Errors:

• [2023-06-23T07:13:26.354Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1142 (message):