Fix missing wx/timer.h header compile issue

Tried to compile the latest git version on my system and got an error that wxTimer could not be found. Fixed it by adding in the header.

cc @kocikdav

src/slic3r/GUI/Preferences.hpp

@@ -5,6 +5,7 @@
 #include "GUI_Utils.hpp"
 
 #include <wx/dialog.h>
+#include <wx/timer.h>
 #include <map>
 
 class wxColourPickerCtrl;