BaseTools: Fix a bug about list the PCD in "not used" section

BaseTools: Fix a bug about list the PCD in "not used" section

Defined a pcd in Ovmf.dec and used that pcd in AcpiPlatformDxe.inf,
then assign a value to that pcd from DSC, then build Ovmf platform
successfully. But this Pcd was wrongly listed into not used section
in the report.txt file.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>