Fix Windows build: tell the build scripts that libsvn_subr has a new private header.
Fix Windows build: tell the build scripts that libsvn_subr has a new
private header.

* build.conf
  (libsvn_sub): add private\svn_string_private.h to exports

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1130598 13f79535-47bb-0310-9956-ffa450edef68

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