arm64: efi: add efi_handle_corrupted_x18 prototype

This functions is only called from assembler and lacks a prototype,
which is seen from this W=1 warning:

arch/arm64/kernel/efi.c:155:25: error: no previous prototype for 'efi_handle_corrupted_x18' [-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/20230516160642.523862-9-arnd@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/efi.h

@@ -166,4 +166,6 @@ static inline void efi_capsule_flush_cache_range(void *addr, int size)
 	dcache_clean_inval_poc((unsigned long)addr, (unsigned long)addr + size);
 }
 
+efi_status_t efi_handle_corrupted_x18(efi_status_t s, const char *f);
+
 #endif /* _ASM_EFI_H */