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

include/radiotap.h

@@ -37,6 +37,11 @@ namespace Tins {
      */
     class RadioTap : public PDU {
     public:
+        /**
+         * \brief This PDU's flag.
+         */
+        static const PDU::PDUType pdu_flag = PDU::RADIOTAP;
+    
         /**
          * \brief Enumeration of the different channel type flags.
          *