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

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion