Fix a compatibility problem

It turns out that my attempt at handling Super, Hyper and Meta better
is causing problems, mostly because Alt and Meta are commonly colocated
in the modmap, and apps do a check for the Alt modifier regularly.

See e.g bug 607697.

gdk/win32/gdkkeys-win32.c

@@ -872,6 +872,12 @@ gdk_keymap_translate_keyboard_state (GdkKeymap       *keymap,
   return tmp_keyval != GDK_VoidSymbol;
 }
 
+void
+gdk_keymap_add_virtual_modifiers (GdkKeymap       *keymap,
+                                  GdkModifierType *state)
+{
+}
+
 gboolean
 gdk_keymap_map_virtual_modifiers (GdkKeymap       *keymap,
                                   GdkModifierType *state)