Follow-up to r1592724: Fix an improper format string.

Found by: julianfoad

* subversion/svn/propget-cmd.c
  (svn_opt_private.h): Include.
  (svn_cl__propget): Stringify START_REVISION correctly.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1593654 13f79535-47bb-0310-9956-ffa450edef68