BaseTools: Guid.xref contain information from FILE statements in FDF

Update Guid.xref to contain information from FILE statements in FDF
file.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=778


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