arm64: add alt_cb_patch_nops prototype

arm64: add alt_cb_patch_nops prototype

alt_cb_patch_nops() is called through an inline asm macro, so
it does not need a prototype for the caller, but adding it
avoids this W=1 build warning:

arch/arm64/kernel/alternative.c:295:14: error: no previous prototype for 'alt_cb_patch_nops' [-Werror=missing-prototypes]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230516160642.523862-16-arnd@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>