Apparently MSC doesn't understand implicit forward declarations.

* subversion/include/svn_types.h
  (struct svn_null_pointer_constant_stdarg_sentinel_t): Forward-delcare the
   struct used by the SVN_VA_NULL macro.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1536308 13f79535-47bb-0310-9956-ffa450edef68