efi: libstub: Give efi_main() asmlinkage qualification

To stop the bots from sending sparse warnings to me and the list about
efi_main() not having a prototype, decorate it with asmlinkage so that
it is clear that it is called from assembly, and therefore needs to
remain external, even if it is never declared in a header file.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/x86-stub.c

@@ -765,7 +765,7 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle)
  * relocated by efi_relocate_kernel.
  * On failure, we exit to the firmware via efi_exit instead of returning.
  */
-unsigned long efi_main(efi_handle_t handle,
+asmlinkage unsigned long efi_main(efi_handle_t handle,
 				  efi_system_table_t *sys_table_arg,
 				  struct boot_params *boot_params)
 {