Fixed compile errors from lack of "override" keyword.

src/google/protobuf/compiler/mock_code_generator.h

@@ -108,7 +108,7 @@ class MockCodeGenerator : public CodeGenerator {
 
   virtual bool Generate(const FileDescriptor* file,
                         const std::string& parameter, GeneratorContext* context,
-                        std::string* error) const;
+                        std::string* error) const override;
 
   uint64 GetSupportedFeatures() const override;
   void SuppressFeatures(uint64 features);

src/google/protobuf/compiler/python/python_generator.h

@@ -72,7 +72,7 @@ class PROTOC_EXPORT Generator : public CodeGenerator {
   virtual bool Generate(const FileDescriptor* file,
                         const std::string& parameter,
                         GeneratorContext* generator_context,
-                        std::string* error) const;
+                        std::string* error) const override;
 
   uint64 GetSupportedFeatures() const override;