Changed go_package for plugin.proto.

For more info see: https://github.com/google/protobuf/issues/1289

src/google/protobuf/compiler/plugin.proto

@@ -49,7 +49,7 @@ package google.protobuf.compiler;
 option java_package = "com.google.protobuf.compiler";
 option java_outer_classname = "PluginProtos";
 
-option go_package = "plugin_go";
+option go_package = "github.com/golang/protobuf/protoc-gen-go/plugin;plugin_go";
 
 import "google/protobuf/descriptor.proto";