MINOR: hlua: Add regex class

This patch simply brings HAProxy internal regex system to the Lua API.
Lua doesn't embed regexes, now it inherits from the regexes compiled
with haproxy.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index bff29d0..74dcf00 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -24,6 +24,7 @@
 #define CLASS_PROXY        "Proxy"
 #define CLASS_SERVER       "Server"
 #define CLASS_LISTENER     "Listener"
+#define CLASS_REGEX        "Regex"
 
 struct stream;