Make sure all of the license headers for IRCv3 are GPLv2

Testing Done:
Ran `licensecheck` from debian in the following way.

```
$ licensecheck *.[ch] | cut -d: -f 2 | sort | uniq -c
27 GNU General Public License v2.0 or later
```

Reviewed at https://reviews.imfreedom.org/r/2913/