Update unknown_field_checker.h - Add using Declaration

Using a `using` declaration in the derived class to bring the base class's method into the derived class's scope.

Author: dchakarwarti

proto_processing_lib/proto_scrubber/unknown_field_checker.h

@@ -36,6 +36,8 @@ class UnknownFieldCheckerTestPeer;
 // message.
 class UnknownFieldChecker : public FieldCheckerInterface {
  public:
+  using FieldCheckerInterface::CheckField;
+  
   // This type is neither copyable nor movable.
   UnknownFieldChecker(const UnknownFieldChecker&) = delete;
   UnknownFieldChecker& operator=(const UnknownFieldChecker&) = delete;