On svn-auth-x509 branch, add missing includes.

* subversion/include/svn_x509.h: Need apr_pools.h and svn_error.h


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1606187 13f79535-47bb-0310-9956-ffa450edef68