[FIX] rviz_default_plugins package deps

[wodtko]

image_transport switched to modern CMake style.
by this change, a missing dependency export led to errors while linking the rviz_default_plugin package to other custom rviz_plugins.
I.e., the linker was not able to find -limage_transport::image_transport.
CMake resolves the issue and finds the package if it is added to the dependency list of rviz_default_plugins.

[iuhilnehc-ynos]

Fixed in the #813

[clalancette]

Closing as a duplicate of the (now merged) #813