cogl-gst-plugin: Fix the license name in the plugin description

In commit 1b83ef938f the license in the plugin description was changed
from “LGPL” to “MIT”. GStreamer strictly whitelists the names of the
licenses and the correct name for the MIT license is “MIT/X11” so it
was rejecting the plugin.

Reviewed-by: Robert Bragg <robert.bragg@intel.com>