Remove an unused variable

class.c

@@ -1272,7 +1272,7 @@ static int
 do_include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super, bool check_cyclic)
 {
     VALUE p, iclass, origin_stack = 0;
-    int method_changed = 0, add_subclass;
+    int method_changed = 0;
     long origin_len;
     VALUE klass_origin = RCLASS_ORIGIN(klass);
     VALUE original_klass = klass;