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)
This is okay with the C++ standard, although you will get a "pure virtual function call" error during runtime.

Nothing prevents you from implementing 'FramePositionTrackingViewController::reset' and calling explicitly. The pure virtual declaration just means it doesn't have an entry in the vtable. See e.g. http://blogs.msdn.com/b/oldnewthing/archive/2013/10/11/10455907.aspx.

Maybe you get a warning on Linux and a compile error on MacOS nonetheless. This should be removed in any case.