BUILD: dns: include tools.h in dns.c

It is used for get_addr_len() without being included. It could be worth
splitting address manipulation functions to a different set of files.
diff --git a/src/dns.c b/src/dns.c
index a6b8fa7..083850d 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -33,6 +33,7 @@
 #include <haproxy/ring.h>
 #include <haproxy/stream.h>
 #include <haproxy/stream_interface.h>
+#include <haproxy/tools.h>
 
 static THREAD_LOCAL char *dns_msg_trash;