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.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>