Fix ffmpeg includes to match the install of the platform

[gpotts]

The ffmpeg for predator video we normalized all platforms to require a ffmpeg prefix. The installer of other platforms like mingw,and unix/linux based systems do something like include/libavcodec include/libavutil ... etc so for these platforms the current svn now uses that style of include. It appears though that their MAC configure does a install of the form include/ffmpeg so for now I have a conditional in ossimffmpegApi.h file in Predator module.