package/gstreamer1/gst1-plugins-good: add optional support for libv4l

package/gstreamer1/gst1-plugins-good: add optional support for libv4l

When libv4l was compiled before, gst1-plugins-good will use it as
optional dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/gstreamer-1.0/libgstvideo4linux2.so | grep NEEDED | sort
[...]
 0x0000000000000001 (NEEDED)             Shared library: [libv4l2.so.0]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>