Guard closing capture window with ifdef.

That variable is only defined if we HAVE_LIBPCAP. Should fix OSX10.5 buildbot.

Change-Id: I0bafc48955ef4af3c0b8d9d7a35b1e8b27577d31
Reviewed-on: https://code.wireshark.org/review/1669
Reviewed-by: Evan Huus <eapache@gmail.com>

ui/qt/main_window.cpp

@@ -409,7 +409,9 @@ void MainWindow::closeEvent(QCloseEvent *event) {
         return;
     }
 
+#ifdef HAVE_LIBPCAP
     capture_interfaces_dialog_.close();
+#endif
     // Make sure we kill any open dumpcap processes.
     delete main_welcome_;