[thread] Add missing explicit template instantiation

Now added for symmetry with surrounding code.

core/thread/src/TReentrantRWLock.cxx

@@ -392,4 +392,5 @@ template class TReentrantRWLock<std::mutex, ROOT::Internal::RecurseCounts>;
 
 template class TReentrantRWLock<ROOT::TSpinMutex, ROOT::Internal::UniqueLockRecurseCount>;
 template class TReentrantRWLock<TMutex, ROOT::Internal::UniqueLockRecurseCount>;
+template class TReentrantRWLock<std::mutex, ROOT::Internal::UniqueLockRecurseCount>;
 }