nm-applet: Install glib schema

Unless we inherit gsettings class, glib schema does not get installed and
users cannot launch nm-applet:

	GLib-GIO-ERROR **: Settings schema 'org.gnome.nm-applet' is not
installed

Signed-off-by: Adam YH Lee <adam.yh.lee@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>