gtk: Use the right macro when defining new Gtk type

Using G_DEFINE_TYPE() instead of G_DEFINE_TYPE_WITH_CODE() when
there is no extra code to declare removes build warnings:

src/gtktechnology.c: In function ‘gtk_technology_get_type’:
src/gtktechnology.c:42:809: warning: statement with no effect [-Wunused-value]
src/gtkservice.c: In function ‘gtk_service_get_type’:
src/gtkservice.c:44:764: warning: statement with no effect [-Wunused-value]

Reported by Daniel Wagner