efi: Export symbol efi_mem_desc_lookup

Building efidrm as module requires efi_mem_desc_lookup(). Export
the symbol.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250408091837.407401-2-tzimmermann@suse.de