selftest: af_unix: Silence -Wflex-array-member-not-at-end warning for scm_rights.c.

selftest: af_unix: Silence -Wflex-array-member-not-at-end warning for scm_rights.c.

scm_rights.c has no problem in functionality, but when compiled with
-Wflex-array-member-not-at-end, it shows this warning:

scm_rights.c: In function ‘__send_fd’:
scm_rights.c:275:32: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
  275 |                 struct cmsghdr cmsghdr;
      |                                ^~~~~~~

Let's silence it.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250811215432.3379570-4-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>