Added a constant static value to each PDU indicating the corresponding PDU::PDUType flag.

include/tcp.h

@@ -44,6 +44,11 @@ namespace Tins {
 
     class TCP : public PDU {
     public:
+        /**
+         * \brief This PDU's flag.
+         */
+        static const PDU::PDUType pdu_flag = PDU::TCP;
+    
         /**
          * \brief TCP flags enum.
          *