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

A number of functions from tools.h are used there but the file was not
included.
diff --git a/src/vars.c b/src/vars.c
index 572b5f5..996141f 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -15,6 +15,7 @@
 #include <haproxy/stream-t.h>
 #include <haproxy/tcp_rules.h>
 #include <haproxy/tcpcheck.h>
+#include <haproxy/tools.h>
 #include <haproxy/vars.h>