modules: mark __inittest/__exittest as __maybe_unused

clang warns about unused inline functions by default:

arch/arm/crypto/aes-cipher-glue.c:68:1: warning: unused function '__inittest' [-Wunused-function]
arch/arm/crypto/aes-cipher-glue.c:69:1: warning: unused function '__exittest' [-Wunused-function]

As these appear in every single module, let's just disable the warnings by marking the
two functions as __maybe_unused.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jessica Yu <jeyu@redhat.com>