efi: Pull up arch-specific prototype efi_systab_show_arch()

Pull up arch-specific prototype efi_systab_show_arch() in order to
fix a -Wmissing-prototypes warning:

arch/x86/platform/efi/efi.c:957:7: warning: no previous prototype for
‘efi_systab_show_arch’ [-Wmissing-prototypes]
char *efi_systab_show_arch(char *str)
Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Link: https://lore.kernel.org/r/20200516132647.14568-1-b.thiel@posteo.deSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/efi.c

@@ -130,11 +130,8 @@ static ssize_t systab_show(struct kobject *kobj,
 	if (efi.smbios != EFI_INVALID_TABLE_ADDR)
 		str += sprintf(str, "SMBIOS=0x%lx\n", efi.smbios);
 
-	if (IS_ENABLED(CONFIG_IA64) || IS_ENABLED(CONFIG_X86)) {
-		extern char *efi_systab_show_arch(char *str);
-
+	if (IS_ENABLED(CONFIG_IA64) || IS_ENABLED(CONFIG_X86))
 		str = efi_systab_show_arch(str);
-	}
 
 	return str - buf;
 }

include/linux/efi.h

@@ -1245,4 +1245,6 @@ struct linux_efi_memreserve {
 
 void __init efi_arch_mem_reserve(phys_addr_t addr, u64 size);
 
+char *efi_systab_show_arch(char *str);
+
 #endif /* _LINUX_EFI_H */