testsuite: g++.dg: vect: fix PR37143 filename

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

gcc/testsuite/ChangeLog:
	PR tree-optimization/37143

	* g++.dg/vect/pr37143.C: Move to...
	* g++.dg/vect/pr37143.cc: ...here.

diff --git a/gcc/testsuite/g++.dg/vect/pr37143.C b/gcc/testsuite/g++.dg/vect/pr37143.cc
similarity index 100%
rename from gcc/testsuite/g++.dg/vect/pr37143.C
rename to gcc/testsuite/g++.dg/vect/pr37143.cc