Fix rb_thread_create argument type

in response to the declaration change in
e3fc30564e9466d6926f9d25a090dcf787bd5c33.

Fixing the AppVeyor mswin CI failure:
https://ci.appveyor.com/project/ruby/ruby/builds/26980881/job/2j6h1qwjnbc8cpop

ref: ruby/ruby#2404

optional/capi/ext/thread_spec.c

@@ -125,7 +125,7 @@ static VALUE thread_spec_rb_thread_create(VALUE self, VALUE proc, VALUE arg) {
   rb_ary_push(args, proc);
   rb_ary_push(args, arg);
 
-  return rb_thread_create(thread_spec_call_proc, (void*)args);
+  return rb_thread_create((VALUE (*)(void *))thread_spec_call_proc, (void*)args);
 }