dnsproxy: Free gresolv on exit

valgrind reported a leak in __connman_wpad_start(). Though the resolv
object will be reused and therefore valgrind is reported the wrong
leaker. dnsproxy happely allocates the resolver but never releases it.

connman/src/dnsproxy.c

@@ -4031,4 +4031,9 @@ void __connman_dnsproxy_cleanup(void)
 	g_hash_table_destroy(listener_table);
 
 	g_hash_table_destroy(partial_tcp_req_table);
+
+	if (ipv4_resolve)
+		g_resolv_unref(ipv4_resolve);
+	if (ipv6_resolve)
+		g_resolv_unref(ipv6_resolve);
 }