Follow-up to r1520419. Fix compiler warning.

* subversion/libsvn_repos/dump.c
  (serialize_node_kind,
   deserialize_node_kind): mark static functions as such

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520433 13f79535-47bb-0310-9956-ffa450edef68