ffmpeg: fix static linking build failure when using libavutil

If a package tries to static link with libavutil it fails due to the
lack of libavutil private dependencies in libavutil.pc (-ldrm in this
case).

Add patch to:
- Check if libdrm is present.
- Add it to Libs.private: in libavutil.pc if present.

Fixes:
http://autobuild.buildroot.net/results/766/766de487f394490df8c712652ac364ebb4a3ab14/
http://autobuild.buildroot.net/results/041/041e29dfddb2da3309ac7d34a576c60c5a75fe4d/
http://autobuild.buildroot.net/results/780/78061b61cfe3f42554a475c048d54dacacfe11d5/
http://autobuild.buildroot.net/results/275/275e4e0030d26c029085b408cfb272d5633969c6/
http://autobuild.buildroot.net/results/515/5152dcca58944cf732d09fba6e6c9af8a9243c75/
http://autobuild.buildroot.net/results/395/395be1a9cab824b82ef34c2ebd84d54243029b33/



Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>