scroll-view: Support GTK_POLICY_EXTERNAL

GTK+ added a new PolicyType which currently triggers compiler warnings
about unhandled values in switch statements. We also have a use case for
it already, so add support for the new policy type.

https://bugzilla.gnome.org/show_bug.cgi?id=739379

tests/interactive/scroll-view-sizing.js

@@ -312,6 +312,9 @@ function test() {
 	    button.label = 'NEVER';
 	    break;
         case 'NEVER':
+	    button.label = 'EXTERNAL';
+	    break;
+        case 'EXTERNAL':
 	    button.label = 'AUTOMATIC';
 	    break;
         }