pidgin/pidgin

Install our appdata file to $(datadir)/metainfo as that change was made a long time ago

This patch originates from Lars Wendler from Gentoo

https://gitweb.gentoo.org/repo/gentoo.git/commit/net-im/pidgin/files/pidgin-2.13.0-metainfo.patch?id=2dd6c1852251b5b67c3045badf173be9535b97a9

Testing Done:
Installed to a prefix and verified the file ended up in `$prefix/share/metainfo`

Reviewed at https://reviews.imfreedom.org/r/607/
For information on writing a plugin for Purple, Pidgin or Finch, go
http://developer.pidgin.im and click on API. From there, see the
HOWTOs in the "Related Pages" section.
You can also generate this documentation locally by installing
doxygen and graphviz dot, then running "make docs" in the
source tree. The documentation will be in the docs/html directory.