core: build keyfile plugin into NetworkManager

Since settings storage is now handled by NetworkManager, we must
have the ability to read/write all connection types at all times.
Since the 'keyfile' plugin is the only plugin that can handle all
connection types, build it into NetworkManager.

system-settings/plugins/keyfile/plugin.h

@@ -43,4 +43,6 @@ GType sc_plugin_keyfile_get_type (void);
 
 GQuark keyfile_plugin_error_quark (void);
 
+GObject *nm_settings_keyfile_plugin_new (void);
+
 #endif	/* _PLUGIN_H_ */