contrib: Set pkg-config path in ffmpeg configure

Although we know in the contrib build system the path to pkg-config,
ffmpeg does not always pick it up and dependency resolution will
fail.
Acked-by: Rémi Denis-Courmont <remi@remlab.net>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/ffmpeg/rules.mak

@@ -14,6 +14,7 @@ endif
 
 FFMPEGCONF = \
 	--cc="$(CC)" \
+	--pkg-config="$(PKG_CONFIG)" \
 	--disable-doc \
 	--disable-encoder=vorbis \
 	--enable-libgsm \