In commit 38747a27f36dbaadadb1399ec085d88e48c97555, the package switched from
a mirror:// URL to a ftp:// URL, but there doesn't appear to have been any
particular reason for that.
* gnu/packages/gnupg.scm (gnupg)[source]{uri}: Use a mirror:// again.
Signed-off-by: Marius Bakke <marius@gnu.org>