sunrpc: Suppress clang -Wgnu-variable-sized-type-not-at-end warning on struct cmessage

sunrpc: Suppress clang -Wgnu-variable-sized-type-not-at-end warning on struct cmessage

clang issues:

svc_unix.c:318:18: error: field 'cmsg' with variable sized type 'struct cmsghdr' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
  318 |   struct cmsghdr cmsg;
      |                  ^

The __msgread explicitly expects that 'struct ucred' is after the 'cmsg',
so suppress the warning.

Reviewed-by: Sam James <sam@gentoo.org>