Add private sqlite api to enable the sqlite logging infrastructure in an
attempt to find out why some tests keep failing on the Windows buildbot.

* subversion/include/private/svn_sqlite.h
  (svn_sqlite__dbg_enable_errorlog): New function.

* subversion/libsvn_subr/sqlite.c
  (sqlite_error_log,
   svn_sqlite__dbg_enable_errorlog): New function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1558468 13f79535-47bb-0310-9956-ffa450edef68