package/libevent: Fix broken download URL

package/libevent: Fix broken download URL

Upstream URL has changed on Sourceforge:

>>> libevent 2.0.22-stable Downloading
--2016-01-17 23:06:31--  http://downloads.sourceforge.net/project/levent/libevent/libevent-2.0/libevent-2.0.22-stable.tar.gz
Resolving downloads.sourceforge.net (downloads.sourceforge.net)... 216.34.181.59
Connecting to downloads.sourceforge.net (downloads.sourceforge.net)|216.34.181.59|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2016-01-17 23:06:31 ERROR 404: Not Found.

Additionally the project moved to github:
http://sourceforge.net/projects/levent/

"As of 2015-10-07, this project may now be found at
 https://github.com/libevent/libevent."

This patch points Config.in to the new upstream URL and fixes the broken
download URL by using github.com.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>