Add <bits/types/res_state.h> to simplify backports.
Introduced upstream as part of this commit:
commit 500b3a499fff61157db464a99f459c772d7eb6c0
Author: Zack Weinberg <zackw@panix.com>
Date: Wed Mar 22 09:55:25 2017 -0400
Remove __need_list_t and __need_res_state.
diff --git a/include/bits/types/res_state.h b/include/bits/types/res_state.h
new file mode 100644
index 0000000000000000..172103878d28a7ee
--- /dev/null
+++ b/include/bits/types/res_state.h
@@ -0,0 +1 @@
+#include <resolv.h>