cpufeature: avoid warning when compiling with clang

The table id (second) argument to MODULE_DEVICE_TABLE is often
referenced otherwise. This is not the case for CPU features. This
leads to warnings when building the kernel with Clang:
  arch/arm/crypto/aes-ce-glue.c:450:1: warning: variable
    'cpu_feature_match_AES' is not needed and will not be emitted
    [-Wunneeded-internal-declaration]
  module_cpu_feature_match(AES, aes_init);
  ^

Avoid warnings by using __maybe_unused, similar to commit 1f318a8bafcf
("modules: mark __inittest/__exittest as __maybe_unused").

Fixes: 67bad2fdb754 ("cpu: add generic support for CPU feature based module autoloading")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/linux/cpufeature.h

@@ -45,7 +45,7 @@
  * 'asm/cpufeature.h' of your favorite architecture.
  */
 #define module_cpu_feature_match(x, __initfunc)			\
-static struct cpu_feature const cpu_feature_match_ ## x[] =	\
+static struct cpu_feature const __maybe_unused cpu_feature_match_ ## x[] = \
 	{ { .feature = cpu_feature(x) }, { } };			\
 MODULE_DEVICE_TABLE(cpu, cpu_feature_match_ ## x);		\
 								\