BaseTools: include variable namespace GUIDs of HII PCDs in Guid.xref

BaseTools: include variable namespace GUIDs of HII PCDs in Guid.xref

[PcdsDynamicHii]
gFooTokenSpaceGuid.PcdBar|L"Variable"|gVarNameSpaceGuid|0x0|FALSE|NV,BS

This patch add the variable namespace GUIDs in "Guid.xref" that are
used with dynamic HII PCDs.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=452
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>