mediastreamer: fix avcodec functions conflict

Autotools miss avcodec function check when linking statically, leading to
conflicts between local functions and avcodec functions.

Add patch to swap $FFMPEG_LIBS and -lavutil which are checked when
AC_CHECK_LIB on avcodec_* functions. $FFMPEG_LIBS contain -ldrm that must
be listed after -lavutil.

Fixes:
http://autobuild.buildroot.net/results/394/3945e06ea0dd1e16013184fbab5b67b3561c87ce/
http://autobuild.buildroot.net/results/576/576c7d71313c45753848462717200b2b8ff5bb0e/
http://autobuild.buildroot.net/results/f33/f339ac6ea30815eeb8ecb144c971f56c06a9f995



[Peter: adjust commit message to clarify that this is for static linking]
Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>