add ifdef to fix compilation on ubuntu 12.04

wireless.cpp

@@ -550,7 +550,10 @@ int Wireless::nl80211_interface_cb(struct nl_msg *msg, void *arg) {
     case NL80211_IFTYPE_MESH_POINT: { *(callback->iftypeString) = "Mesh Point"; break; }
     case NL80211_IFTYPE_P2P_CLIENT: { *(callback->iftypeString) = "P2P Client"; break; }
     case NL80211_IFTYPE_P2P_GO: { *(callback->iftypeString) = "P2P Go"; break; }
+#if defined(NL80211_IFTYPE_P2P_DEVICE)
+    // ubuntu 12.04 doesn't define this type
     case NL80211_IFTYPE_P2P_DEVICE: { *(callback->iftypeString) = "P2P Device"; break; }
+#endif
     default: { *(callback->iftypeString) = "Unspecified"; break; }
   }