Compile fix for OSX

Compile fix for OSX. I'm not sure where `FramePositionTrackingViewController::reset();` was coming from, I didn't see it defined anywhere..

ps. I don't have it working yet, but this is most likely due to a rviz bug (https://github.com/ros-visualization/rviz/issues/646)
Thanks @dgossow, I figured something like that, but the point I wanted to confirm was that removing this reset was the right thing todo (as opposed to inheriting from something else and calling that non-virtual reset). Sounds like that is the case, @hgaiser can you just remove this line entirely?