x86/efi: Unexport add_efi_memmap variable

This variable was accidentally exported, even though it's only used in
this compilation unit and only during initialization.

Remove the bogus export, make the variable static instead and mark it
as __initdata.

Fixes: 200001eb ("x86 boot: only pick up additional EFI memmap...")
Cc: Paul Jackson <pj@sgi.com>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>