Update CMakeLists.txt

This is an update to the pull request https://github.com/ros-visualization/rviz/pull/825

Moves find package logic up to the main CMakeLists.txt so both image_view and rviz can find it. 

Addresses issue https://github.com/ros-visualization/rviz/issues/824
I believe the section you introduced must come after this line because it uses the `pkg_check_modules(...)` macro.