testsuite: g++.dg: fix PR112938 filename

The test was being ignored because dg.exp looks for .C in g++.dg/.

gcc/testsuite/ChangeLog:
	PR middle-end/112938

	* g++.dg/strub-internal-pr112938.cc: Move to...
	* g++.dg/strub-internal-pr112938.C: ...here.

diff --git a/gcc/testsuite/g++.dg/strub-internal-pr112938.cc b/gcc/testsuite/g++.dg/strub-internal-pr112938.C
similarity index 100%
rename from gcc/testsuite/g++.dg/strub-internal-pr112938.cc
rename to gcc/testsuite/g++.dg/strub-internal-pr112938.C