Merge pull request #3960 from acozzette/libprotoc-export-fix

Added back in LIBPROTOC_EXPORT which was removed by mistake

src/google/protobuf/compiler/code_generator.h

@@ -166,7 +166,7 @@ typedef GeneratorContext OutputDirectory;
 //   "foo=bar,baz,qux=corge"
 // parses to the pairs:
 //   ("foo", "bar"), ("baz", ""), ("qux", "corge")
-void ParseGeneratorParameter(
+LIBPROTOC_EXPORT void ParseGeneratorParameter(
     const string&, std::vector<std::pair<string, string> >*);
 
 }  // namespace compiler