Add GPLv2+ COPYING file as there's a few files still licensed as GPL-2+

This is still code from Marcin Ziemiński but is using a valid license
so I'm less sure I could just relicense it. I think adding the COPYING file
would be acceptable to Debian FTP-Master, or we could relicense the
code to be LGPL-2.1+.

KTp/OTR/channel-adapter.cpp     GPL (v2 or later)
KTp/OTR/channel-adapter.h       GPL (v2 or later)
KTp/OTR/utils.cpp       GPL (v2 or later)
KTp/OTR/utils.h GPL (v2 or later)

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