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)
No idea. I would guess that removing it is the correct action, but I'm confused as to why this compiles on Linux.

@tfoote @dgossow any ideas?