glib2: In Rubinius 2.2.x, `rb_cMutex` is defined as macro.

ref: https://github.com/rubinius/rubinius/blob/04f38de0f0644ba6c099fb3675e8ae2e8f73d08d/vm/include/capi/ruby/ruby.h#L388
glib2: remove case divied with FL_TEST, FL_SINGLETON

It changes always call superclass.
glib2: Indent