libv4lconvert: add support for extended controls

libv4lconvert: add support for extended controls

libv4lconvert did not support the extended control API so qv4l2, which
uses it, didn't work properly with libv4l2 since passing software
emulated controls using the extended control API will fail as those
controls are obviously not known to the driver.

This patch adds support for this API.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed by: Hans de Goede <hdegoede@redhat.com>