Follow-up to r1245284:
Follow-up to r1245284:

* subversion/libsvn_client/merge.c: Include svn_subst.h.

Found by: Windows Buildbot

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