add DEFAULT_UPNP constant in net

- as this is a shared Core/GUI setting, this makes it easier to keep them
  in sync (also no new includes are needed)

src/net.h

@@ -38,6 +38,12 @@ namespace boost {
 
 /** The maximum number of entries in an 'inv' protocol message */
 static const unsigned int MAX_INV_SZ = 50000;
+/** -upnp default */
+#ifdef USE_UPNP
+static const bool DEFAULT_UPNP = USE_UPNP;
+#else
+static const bool DEFAULT_UPNP = false;
+#endif
 
 inline unsigned int ReceiveFloodSize() { return 1000*GetArg("-maxreceivebuffer", 5*1000); }
 inline unsigned int SendBufferSize() { return 1000*GetArg("-maxsendbuffer", 1*1000); }