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

The file session.c calls plenty of functions from tools.h but did not
include it.
diff --git a/src/session.c b/src/session.c
index 1038a36..0f26588 100644
--- a/src/session.c
+++ b/src/session.c
@@ -20,6 +20,7 @@
 #include <haproxy/proxy.h>
 #include <haproxy/session.h>
 #include <haproxy/tcp_rules.h>
+#include <haproxy/tools.h>
 #include <haproxy/vars.h>