vm_method.c: suppress warnings

* vm_method.c (call_cfunc_invoker_func): suppress wrong warnings
  "C4550: expression evaluates to a function which is missing an
  argument list."  by VC6.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38847 b2dd03c8-39d4-4d8f-98ff-823fe69b080e