Try to fix compilation on OS X.

Change-Id: I63d1f639bf5fd56aeaea112c797efedca8c0bf6e
Reviewed-on: https://code.wireshark.org/review/7090
Reviewed-by: Gerald Combs <gerald@wireshark.org>

ui/qt/main_window.h

@@ -56,6 +56,7 @@
 #include "file_set_dialog.h"
 #include "filter_action.h"
 #include "follow_stream_dialog.h"
+#include "preferences_dialog.h"
 
 class ByteViewTab;
 class MainWelcome;