libstdc++: Fix test failure on AIX

libstdc++: Fix test failure on AIX

This fixes a failure on AIX 7.2:

FAIL: 17_intro/names.cc (test for excess errors)
Excess errors:
/home/jwakely/src/gcc/libstdc++-v3/testsuite/17_intro/names.cc:99: error: expected identifier before '(' token
/usr/include/sys/var.h:187: error: expected unqualified-id before '{' token
/usr/include/sys/var.h:187: error: expected ')' before '{' token
/usr/include/sys/var.h:337: error: expected unqualified-id before ';' token
/usr/include/sys/var.h:337: error: expected ')' before ';' token

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/names.cc: Do not test 'v' on AIX.