ANDROID: arm64: Suppress unused function error of cpus_have_cap()

Out-of-tree fips140 modules may include asm/alternative-macros.h but not
using any alternative_has_cap_*() macros, which results in the unused
function error of cpus_have_cap().

For example, asm/alternative-macros.h could be unintentionally included
through the following path:
  linux/bitops.h
  -> asm-generic/bitops/generic-non-atomic.h
  -> asm/barrier.h
  -> asm/alternative-macros.h

Add __maybe_unused to the forward declaration of cpus_have_cap() to
suppress the error.

Fixes: 059d29197b ("ANDROID: arm64: only permit certain alternatives in the FIPS140 module")
Bug: 383224684
Change-Id: I72d2b7e3b2923f29a5cdc524081cd785540db342
Signed-off-by: Jacky Liu <qsliu@google.com>

arch/arm64/include/asm/alternative-macros.h

@@ -302,7 +302,7 @@ l_yes:
 
 #include <linux/types.h>
 
-static bool cpus_have_cap(unsigned int num);
+static bool __maybe_unused cpus_have_cap(unsigned int num);
 
 /*
  * Return 'false' for all capabilities listed above, and use the slow path for