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/Library/SecureBootVariableLib/UnitTest/SecureBootVariableLibUnitTest.c

@@ -163,7 +163,7 @@ MockGetVariable (
     return EFI_BUFFER_TOO_SMALL;
   } else {
     assert_non_null (Data);
-    CopyMem (Data, (VOID *)mock (), TargetSize);
+    CopyMem (Data, (VOID *)(UINTN)mock (), TargetSize);
   }
 
   return EFI_SUCCESS;