[packages] ffmpeg: include/disable swresample where appropriate

libswresample is an audio resampling library within ffmpeg.

This patch disables swresample in libffmpeg-custom and libffmpeg-mini and adds
it to libffmpeg-full. Should correct build issue reported in #13089.

Signed-off-by: Ian Leonard <antonlacon@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@35832 3c298f89-4303-0410-b956-a3cf2f4a3e73