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

include/ip.h

@@ -43,6 +43,11 @@ namespace Tins {
      */
     class IP : public PDU {
     public:
+        /**
+         * \brief This PDU's flag.
+         */
+        static const PDU::PDUType pdu_flag = PDU::IP;
+    
         /**
          * \brief IP address size.
          */