rs6000/testsuite: Skip pr105140.c

rs6000/testsuite: Skip pr105140.c

This test fails with error "AltiVec argument passed to unprototyped
function", but the code (in rs6000.c:invalid_arg_for_unprototyped_fn,
from 2005) actually tests for any vector type argument.  It also does
not fail on Darwin, not reflected here though.

2022-04-06  Segher Boessenkool  <segher@kernel.crashing.org>

gcc/testsuite/
	PR target/105147
	* gcc.dg/pr105140.c: Skip for powerpc*-*-*.