lib/efi_loader: fix ABI in efi_mm_communicate_header

lib/efi_loader: fix ABI in efi_mm_communicate_header

Pack struct efi_mm_communicate_header as done in EDK2 as seen in
release 201808 [1]. If not packed sizeof() for the structure adds
4 additional bytes on 32bit targets which breaks the ABI.

Link: [1] https://github.com/tianocore/edk2/blob/edk2-stable201808/MdePkg/Include/Protocol/MmCommunication.h#L21
Fixes: 23a397d2e2fb ("efi_loader: Add headers for EDK2 StandAloneMM communication")
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>