BaseTools: Support Structure PCD value assignment in DEC/DSC

https://bugzilla.tianocore.org/show_bug.cgi?id=542
This is pure BaseTools enhancement to support PCD with one structure.
User can specify PCD value based on its structure field.

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

BaseTools/Source/Python/Workspace/WorkspaceCommon.py

@@ -15,6 +15,7 @@ from Common.Misc import sdict
 from Common.DataType import SUP_MODULE_USER_DEFINED
 from BuildClassObject import LibraryClassObject
 import Common.GlobalData as GlobalData
+from Workspace.BuildClassObject import StructurePcd
 
 ## Get all packages from platform for specified arch, target and toolchain
 #