proc.c: adjust argument type

* proc.c (proc_new): adjust `is_lambda` argument type to
  `rb_vm_make_proc_lambda` to suppress a warning by VC6.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48420 b2dd03c8-39d4-4d8f-98ff-823fe69b080e