Fix a bug which causes build break even if the PCD usages in DSC are correct.

Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Signed-off-by: Yingke Liu <yingke.d.liu@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2-buildtools/code/trunk/BaseTools@2634 c2973edb-eda0-4c78-bc6a-9341b269661f