Add missing LIBRPOTOC_EXPORT.

src/google/protobuf/compiler/code_generator.h

@@ -162,8 +162,8 @@ typedef GeneratorContext OutputDirectory;
 //   "foo=bar,baz,qux=corge"
 // parses to the pairs:
 //   ("foo", "bar"), ("baz", ""), ("qux", "corge")
-extern void ParseGeneratorParameter(const string&,
-            std::vector<std::pair<string, string> >*);
+LIBPROTOC_EXPORT void ParseGeneratorParameter(
+    const string&, std::vector<std::pair<string, string> >*);
 
 }  // namespace compiler
 }  // namespace protobuf