selftest: af_unix: Support compilers without flex-array-member-not-at-end support

selftest: af_unix: Support compilers without flex-array-member-not-at-end support

Fix:

gcc: error: unrecognized command-line option ‘-Wflex-array-member-not-at-end’

by making the compiler option dependent on its support.

Fixes: 1838731f1072c ("selftest: af_unix: Add -Wall and -Wflex-array-member-not-at-end to CFLAGS.")
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://patch.msgid.link/20251205171010.515236-7-linux@roeck-us.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>