BaseTools:PCD value error in structure pcd sku case.

BaseTools:PCD value error in structure pcd sku case.

Defined 2 PCDs(Test4 & Test401) and 2 SKUs(DEFAULT & _),
then set "SKUID_Defines" to ALL, for FixedAtBuild
gEfiStructuredPcdPkgTokenSpaceGuid. Test401 in this case,
its value should get from "Default" SKU, not from "_" SKU,
but we does not set value in SKU "_" in dsc, so Test401
should only display the value get from dec.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhiju.Fan <zhijux.fan@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>