mpeg4: fix double definition of GstVaapiDecoderMpeg4Class.

mpeg4: fix double definition of GstVaapiDecoderMpeg4Class.

This fixes the following issue:

  CC     libgstvaapi_0.10_la-gstvaapidecoder_mpeg4.lo
gstvaapidecoder_mpeg4.c:113: error: redefinition of typedef
'GstVaapiDecoderMpeg4Class'
gstvaapidecoder_mpeg4.c:44: note: previous declaration of
'GstVaapiDecoderMpeg4Class' was here
make[5]: *** [libgstvaapi_0.10_la-gstvaapidecoder_mpeg4.lo] Error 1
make[5]: Leaving directory
`/builddir/build/BUILD/gstreamer-vaapi-0.5.5.1/gst-libs/gst/vaapi'

https://bugzilla.gnome.org/show_bug.cgi?id=705148