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.