contrib: ffmpeg: account for libvpx decoders name change

'libvpx' decoder doesn't exist anymore but we continue to disable it anyway
for people building and older version

contrib/src/ffmpeg/rules.mak

@@ -28,7 +28,7 @@ DEPS_ffmpeg = zlib gsm openjpeg
 
 # Optional dependencies
 ifdef BUILD_ENCODERS
-FFMPEGCONF += --enable-libmp3lame --enable-libvpx --disable-decoder=libvpx
+FFMPEGCONF += --enable-libmp3lame --enable-libvpx --disable-decoder=libvpx --disable-decoder=libvpx_vp8 --disable-decoder=libvpx_vp9
 DEPS_ffmpeg += lame $(DEPS_lame) vpx $(DEPS_vpx)
 else
 FFMPEGCONF += --disable-encoders --disable-muxers