SecurityPkg: Add gmock example

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4389

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Signed-off-by: Chris Johnson <chris.n.johnson@intel.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Oliver Smith-Denny <osde@linux.microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

SecurityPkg/SecurityPkg.dec

@@ -21,6 +21,7 @@
 
 [Includes]
   Include
+  Test/Mock/Include
 
 [LibraryClasses]
   ##  @libraryclass  Provides hash interfaces from different implementations.