testsuite: g++.dg: rename pr105820 test

.c is used for C and .C is for C++. The test was being ignored before.

gcc/testsuite/ChangeLog:
	PR tree-optimization/105820

	* g++.dg/tree-ssa/pr105820.c: Move to...
	* g++.dg/tree-ssa/pr105820.C: ...here.

diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr105820.c b/gcc/testsuite/g++.dg/tree-ssa/pr105820.C
similarity index 100%
rename from gcc/testsuite/g++.dg/tree-ssa/pr105820.c
rename to gcc/testsuite/g++.dg/tree-ssa/pr105820.C