[ros2] link image_transport

[wodtko]

the ros2 feature branch of the image_transport package now creates a modern CMake target instead of setting _INCLUDE_DIRS and _LIBRARIES variables.
Thus, the include of image_transport/image_transport.h fails during the build.

I would suggest creating a new branch, something like rolling-devel or similar, since changes like this might come for other dependency packages.
The respective CMake fix is rather small. (link against new target image_transport::image_transport, or ${image_transport}_TARGETS)

[wodtko]

a respective fix is available at (https://github.com/wodtko/rqt_image_view/tree/rolling-devel) not sure about the new branch tho.
linking ${image_transport}_TARGETS would, in general, link all targets of the package, but the only dependency is image_transport::image_transport, which is why I only added this specific target.

[RFRIEDM-Trimble]

FYI, with this PR, you can now consume the alias targets.
ros-perception/image_common@2edae7c