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 class inherits from `rviz::FramePositionTrackingViewController`:

https://github.com/ros-visualization/oculus_rviz_plugins/blob/groovy-devel/include/oculus_rviz_plugins/fixed_view_controller.h#L47

Was this preventing it from compiling?