MOHAWK: Add missing break in switch, CID 1003750

Clicking on any of the image boxes would fall through to the case
where it checks if you are picking up or putting away the white
page, which just doesn't make any sense. It didn't have any
noticeable impact on the game, because that case only does
something if the hiding place for the white page is open, and
apparently that can only happen while you're at that hiding place.

I have verified that it's still possible to get the white page,
and that it's still possible to raise the sunken ship.

engines/mohawk/myst_stacks/myst.cpp

@@ -691,6 +691,7 @@ void Myst::toggleVar(uint16 var) {
 			else
 				_state.courtyardImageBoxes |= mask;
 		}
+		break;
 	case 41: // Vault white page
 		if (_globals.ending != 4) {
 			if (_dockVaultState == 1) {