[gnome-extra/nm-applet-gtk2] fix compile error

Problem lies in introspection support: 'NMGtk-1.0.gir' includes 'Gtk-3.0.gir' which may not exist on some systems.
This sed command replaces 'Gtk-3.0.gir' with 'Gtk-2.0.gir' which is in the gtk2 package.