checksyscalls: ignore -Wunused-macros

The macros defined in this file are for testing only and are purposely
not used. When compiled with W=2, both gcc and clang yield some
-Wunused-macros warnings. Ignore them.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>