image_view: Add depend on gtk2

GTK2 headers are needed because of https://github.com/ros-perception/image_pipeline/blob/indigo/image_view/CMakeLists.txt#L8

The `rosdep` key `gtk` provides those headers: https://github.com/ros/rosdistro/blob/master/rosdep/base.yaml#L747-L756

Thanks!