Adding explicit cmake policy to rosconsole-extras.cmake

I don't know if this jives with the philosophy with the catkin/ros_comm maintainers but adding this to the `rosconsole-extras.cmake` prevents the user error (incorrect camke_minimum_required version) reported in https://github.com/ros/ros_comm/pull/245 from preventing packages to build. This `cmake_policty()` command should be local to this file only. I chose `cmake_policy()` instead of `cmake_minimum_required()` due to it's clearer meaning.

See the follwoing for reference: http://www.cmake.org/Wiki/CMake/Policies#Fixing_an_Interface_Breaks_Work-Arounds