BUILD: server: include tools.h from server.c

A lot of functions from tools.h are used there but the file was only
inherited via other ones.
diff --git a/src/server.c b/src/server.c
index 75977d8..fc78889 100644
--- a/src/server.c
+++ b/src/server.c
@@ -44,6 +44,7 @@
 #include <haproxy/task.h>
 #include <haproxy/tcpcheck.h>
 #include <haproxy/time.h>
+#include <haproxy/tools.h>
 
 
 static void srv_update_status(struct server *s);