dumm: Fix -Wformat warning in ruby extension

In recent ruby versions, extensions get built with -Wformat. As we use custom
printf specifiers, that does not work for us. As there does not seem to be a
reliable way to override -Wformat, we use a variable for the format string,
which prevents gcc from doing the -Wformat check in that particular situation.

src/dumm/ext/dumm.c

@@ -629,7 +629,7 @@ static VALUE iface_each_addr(int argc, VALUE *argv, VALUE self)
 	linked_list_t *list;
 	iface_t *iface;
 	host_t *addr;
-	char buf[64];
+	char buf[64], *fmt = "%H";
 
 	if (!rb_block_given_p())
 	{
@@ -645,7 +645,7 @@ static VALUE iface_each_addr(int argc, VALUE *argv, VALUE self)
 	enumerator->destroy(enumerator);
 	while (list->remove_first(list, (void**)&addr) == SUCCESS)
 	{
-		snprintf(buf, sizeof(buf), "%H", addr);
+		snprintf(buf, sizeof(buf), fmt, addr);
 		addr->destroy(addr);
 		rb_yield(rb_str_new2(buf));
 	}