testsuite: vect: update unaligned message

testsuite: vect: update unaligned message

gcc.dg/vect/costmodel/ppc/costmodel-vect-31a.c covers ppc variants
that accept and reject misaligned accesses.  The message that it
expects for rejection was removed in the gcc-11 development cycle by
commit r11-1969.  The patch adjusted multiple tests to use the message
introduced in r11-1945, but missed this one.

for  gcc/testsuite/ChangeLog

	* gcc.dg/vect/costmodel/ppc/costmodel-vect-31a.c: Update
	the expected message for the case in which unaligned accesses
	are not allowed.

(cherry picked from commit 404edfce683cb1801b052ee7c8d45d603c392e08)