Track dav_error API changes in mod_dav_svn (so we can compile against Apache 2.4)

Jeff Trawick <trawick@gmail.com> informs us[1] that he's committed some changes
to Apache's trunk (aimed at 2.4) which cause mod_dav_svn not to compile.  Nice
guy that he is, though, he provided a patch to remedy the problem, too[2].

[1] http://subversion.tigris.org/ds/viewMessage.do?dsForumId=462&dsMessageId=2419490
[2] http://people.apache.org/~trawick/svn_err_api.txt