efi_loader: make efi_block_io_guid a global symbol

The GUID of the EFI_BLOCK_IO_PROTOCOL is needed in different code
parts. To avoid duplication make efi_block_io_guid a global symbol.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>