Explicitly check for and link to libavutil

av_free and friends are used in the ffmpeg plugin, and after r13528
ffmpeg's .pc files do not tell anymore to link needlessly to its
private library requirements.
Signed-off-by: Rémi Denis-Courmont <rem@videolan.org>

configure.ac

@@ -3000,7 +3000,7 @@ if test "${enable_ffmpeg}" != "no"
 then
 
 dnl Trying with pkg-config
-   PKG_CHECK_MODULES(FFMPEG,[libavcodec, libavformat],
+   PKG_CHECK_MODULES(FFMPEG,[libavcodec, libavformat, libavutil],
     [
      VLC_SAVE_FLAGS
      CPPFLAGS="${CPPFLAGS} ${FFMPEG_CFLAGS}"