StandaloneMmPkg: Fix ECC error 10014 in StandaloneMmCpu

Bugzilla: 3150 (https://bugzilla.tianocore.org/show_bug.cgi?id=3150)

Fix ECC error "[10014] No used module files found. The source file
[StandaloneMmCpu.h] is existing in module directory but it is not
described in INF file."

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>