Add missing GDK linking to GIR build and examples:

  GISCAN Gtk-3.0.gir
  CCLD   gtk-query-immodules-3.0
./.libs/libgtk-3.so: undefined reference to `gdk_keymap_get_modifier_mask'
./.libs/libgtk-3.so: undefined reference to `gdk_modifier_intent_get_type'
./.libs/libgtk-3.so: undefined reference to `gdk_window_begin_resize_drag_for_device'
./.libs/libgtk-3.so: undefined reference to `gdk_event_triggers_context_menu'
collect2: ld returned 1 exit status

  CCLD   grid-packing
../gtk/.libs/libgtk-3.so: undefined reference to `gdk_keymap_get_modifier_mask'
[...]

https://bugzilla.gnome.org/show_bug.cgi?id=664027