Fixed a bug on video_background_segm.h

When attempting to use BackgroundSubtractorMOG for a project I found a bug. Most methods for both MOG and MOG2 classes where using a method that was removed months prior from my_functions.h
I replaced the missing function for the Ptr<> cast that it was originally doing.

Also, the #ifndef preprocessor directive was using a name used in video_tracking.h; I fixed that too.

Good luck and congratulations on this amazing project.