Revert "ffmpeg: fix static linking build failure when using libavutil"

This reverts commit 483db990.

It was a test commit that was not supposed to be pushed. The patch
doesn't apply to ffmpeg 3.4.5.

Fixes:
  http://autobuild.buildroot.net/results/f3e/f3eaaff9e55ca7c75fe7094547dc2394b1288849

and many more.