Platform/Sgi: Use MmUnblockMemoryLib when SECURE_BOOT_ENABLE
Building the RdV1 platform fails when only providing the
'-D SECURE_BOOT_ENABLE' flag due to MmUnblockMemoryLib missing.
There is only one MmUnblockMemoryLib implementation used,
so unconditionally use this implementation.
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>