SMACK-388 register PingProvider with smack.providers. The static register code was removed with 13406 (http://fisheye.igniterealtime.org/changelog/smack?cs=13406)

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/trunk@13413 b35dd754-fafc-0310-a699-88a17e54d16e