Do not duplicate MJpegContext struct, #include the proper header instead.
This also fixes the warnings:
jpeg_enc.c:342: warning: implicit declaration of function 'ff_mjpeg_encode_init'
jpeg_enc.c:384: warning: implicit declaration of function 'ff_mjpeg_encode_picture_header'
jpeg_enc.c:489: warning: implicit declaration of function 'ff_mjpeg_encode_picture_trailer'
jpeg_enc.c:500: warning: implicit declaration of function 'ff_mjpeg_encode_close'
git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@25545 b3059339-0415-0410-9bf9-f77b7e298cf2