catkin.bbclass: Stage /usr/etc sysroot direcotry 

The CMake generate_messages function from the genmsg package
detects the installed message generators via files in the
/usr/etc/ros/genmsg directory. Stage the /usr/etc directory
manually as it is not a common path.

Fixed #50

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>