build: fix v4l2 build with system installed ffmpeg

Missing ffmpeg/libav build flags in v4l2 Makefile.
This fix the build of this last when deps are installed
on system (i.e. using contrib, where the path doesn't contain
the implementation name).

Change-Id: I966e501ddfdeba95a87ff89ee77faa4938ebe15d
Tuleap: #1058