videocrop: Resurrect logging category.

videocrop: Resurrect logging category.

Fix for a regression from commit 8f1384c9. That commit moved the debug
category definition, as static, into a gstvideocropelement.c, but that
category was used as default, in gstvideocrop.c, so it was never used
at logging, so the debug selector never showed the logs for
videocrop.

This patch move back the category definition into gstvideocrop.c and
leaving the function videocrop_element_init() as a noop.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/1049>