Add GetExtraBarIndex to the restricted environment
 

5.1 added a number API functions related to action bar remapping to the RestrictedEnvironment, including for example HasOverrideActionBar and GetOverrideBarIndex, HasTempShapeshiftActionBar and GetTempShapeshiftBarIndex.

Although HasExtraActionBar was also added in 5.1, the corresponding GetExtraBarIndex was not (so addons can determine if there's an extra action button, and might even iterate GetActionInfo to find out what it is, but aren't directly told its action slot ID); this seems like an oversight.