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>