prefs: Use an unsigned value for META_KEYBINDING_ACTION_NONE

Keybinding actions are unsigned, so it is a tad bit odd to use a
negative value for NONE and rely on implicit casting.
Use 0 instead.

https://bugzilla.gnome.org/show_bug.cgi?id=643111

src/meta/prefs.h

@@ -145,7 +145,7 @@ void     meta_prefs_set_ignore_request_hide_titlebar (gboolean whether);
  */
 typedef enum _MetaKeyBindingAction
 {
-  META_KEYBINDING_ACTION_NONE = -1,
+  META_KEYBINDING_ACTION_NONE,
   META_KEYBINDING_ACTION_WORKSPACE_1,
   META_KEYBINDING_ACTION_WORKSPACE_2,
   META_KEYBINDING_ACTION_WORKSPACE_3,