package/ifplugd: license is GPL-2.0+

package/ifplugd: license is GPL-2.0+

The source files contain the "(at your option) any later version" text and
the website states:

License

This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your option)
any later version.

http://0pointer.de/lennart/projects/ifplugd/

So change the license to GPL-2.0+

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>