mpegtsdemux: Update plugin license field

mpegtsdemux: Update plugin license field

The original code (old mpegtsparse) from which this plugin was based on
was dual-licensed. This allowed usage of the code under any of the
licenses (which including LGPL):

"""
* Alternatively, the contents of this file may be used under the terms of
* the GNU Lesser General Public License Version 2 or later (the "LGPL"),
* in which case the provisions of the LGPL are applicable instead
* of those above. If you wish to allow use of your version of this file only
* under the terms of the LGPL, and not to allow others to
* use your version of this file under the terms of the MPL, indicate your
* decision by deleting the provisions above and replace them with the notice
* and other provisions required by the LGPL. If you do not delete
* the provisions above, a recipient may use your version of this file under
* the terms of the MPL or the LGPL.
"""

When refactored (leading to the creation of this new plugin), I chose all
new code to be LGPL-only (which was allowed for pre-existing code) by removing
the MPL sections.

The headers were all updated, but not the plugin license field. This commit
fixes this.