gtk: fix compilation warning with gtk 3.22.2

gdk_screen_get_width() is deprecated since gtk 3.22.2, use
gdk_monitor_get_geometry() instead if it's available.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 20161026152108.12364-1-berto@igalia.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>