bitlbee: fix linkage and update license

[Moisan]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with HOMEBREW_NO_INSTALL_FROM_API=1 brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
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This linkage check was failing (spotted in #172629)

> brew linkage --cached --test --strict bitlbee
Indirect dependencies with linkage:
  libgpg-error

I also updated the license since the source headers contains

  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.

For example see this file.

[Moisan]

There is another bitlbee pr open for a different issue: #172018

[carlocab]

Thanks!