v4l2object: fix extra-controls leak
v4l2object: fix extra-controls leak

Gst struct v4l2object->extra_controls is created if user sets appropriate
option but it is not freed on destruction of v4l2object.

https://bugzilla.gnome.org/show_bug.cgi?id=773580

hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,hizukiayaka/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good