x86: efi.c declare add_efi_memmap before they get used

Impact: cleanup, avoid sparse warning

Fixes this sparse warning:

  arch/x86/kernel/efi.c:67:5: warning: symbol 'add_efi_memmap' was not declared. Should it be static?
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/efi.h

@@ -90,6 +90,7 @@ extern void __iomem *efi_ioremap(unsigned long addr, unsigned long size);
 
 #endif /* CONFIG_X86_32 */
 
+extern int add_efi_memmap;
 extern void efi_reserve_early(void);
 extern void efi_call_phys_prelog(void);
 extern void efi_call_phys_epilog(void);