minidlna: fix build failure

With the 1.0.26 version bump a new routine was added to find newer
versions of ffmpeg and/or libav on different include paths.
Unfortunately this uses absolute pathnames hence when there's no
matching ffmpeg/libav headers on the host it fails to build. Fixes:
http://autobuild.buildroot.net/results/20a/20a1b3a9f040d2207b6c533a158e9747054ddd2d/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>