Fixed build errors on MacOS.

src/google/protobuf/compiler/cpp/cpp_generator.h

@@ -81,7 +81,8 @@ class PROTOC_EXPORT CppGenerator : public CodeGenerator {
 
   // implements CodeGenerator ----------------------------------------
   bool Generate(const FileDescriptor* file, const std::string& parameter,
-                GeneratorContext* generator_context, std::string* error) const;
+                GeneratorContext* generator_context,
+                std::string* error) const override;
 
   uint64 GetSupportedFeatures() const override {
     // We don't fully support this yet, but this is needed to unblock the tests,

src/google/protobuf/compiler/java/java_generator.h

@@ -58,7 +58,7 @@ class PROTOC_EXPORT JavaGenerator : public CodeGenerator {
 
   // implements CodeGenerator ----------------------------------------
   bool Generate(const FileDescriptor* file, const std::string& parameter,
-                GeneratorContext* context, std::string* error) const;
+                GeneratorContext* context, std::string* error) const override;
 
   uint64 GetSupportedFeatures() const override;