Fix warning of unused variable in roofit

[ikabadzhov]

This Pull request:

Casted variable rc to void in roofit/multiprocess/src/Messenger.cxx to disable the warning.
It is used only in assert statement besides the cast.

Changes or fixes: roofit/multiprocess/src/Messenger.cxx

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos8-multicore/default, ROOT-ubuntu16/nortcxxmod, ROOT-ubuntu2004/soversion, mac1015/python3, mac11/cxx17, windows10/cxx14
How to customize builds

[guitargeek]

You can close this PR, we are already taking care of it: https://github.com/root-project/root/pull/9349/files#diff-03f30bc721fb7eb65916ce4552fb544751ad9dfde16e608227267692c0bc16a4

Whenever you see such warnings in an actively developed part of ROOT you can assume that somebody else will take care of it :)

[guitargeek]

Just and empty change request to block this PR from merging.

[phsft-bot]

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

Errors:

• [2021-12-03T16:24:31.518Z] stderr: error: Failed to merge in the changes.
• [2021-12-03T16:24:36.686Z] CMake Error at /home/sftnight/build/workspace/root-pullrequests-build/rootspi/jenkins/root-build.cmake:1056 (message):

[guitargeek]

This has already been fixed in #9349