On svn-auth-x509 branch, add the svn_x509.h file to the exports.

* build.conf
  (libsvn_subr): add svn_x509.h to msvc-export.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1613372 13f79535-47bb-0310-9956-ffa450edef68