ra_serf: make svn build with serf 0.3 again.
ra_serf: make svn build with serf 0.3 again.

Found by: hwright

* subversion/libsvn_ra_serf/serf.c
  (load_config): Only call serf_config_credentials_callback when building with
   serf >= 0.4.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@894320 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion