update polytypes.c -flax-vector-conversions msg

update polytypes.c -flax-vector-conversions msg

Since commit 2f6d557ff82876432be76b1892c6c3783c0095f4 AKA SVN-r269586,
the inform() message suggesting the use of -flax-vector-conversions
has had quotes around the option name, but the testcase still expected
the message without the quotes.  This patch adds to the expected
compiler output the quotes that are now issues.

for  gcc/testsuite/ChangeLog

	* gcc.target/arm/polytypes.c: Add quotes around
	-flax-vector-conversions.