Fix patch-change bank update loop

Without an object, the Unwinder has no scope and the "ignore" variable
is never set.

In this case: When the patch-bank was changed externally this
lead to a duplicate recursive call to "refill_banks".

gtk2_ardour/patch_change_widget.cc

@@ -229,7 +229,7 @@ PatchChangeWidget::refill_banks ()
 	const int b = bank (_channel);
 
 	{
-		PBD::Unwinder<bool> (_ignore_spin_btn_signals, true);
+		PBD::Unwinder<bool> uw (_ignore_spin_btn_signals, true);
 		_bank_msb_spin.set_value (b >> 7);
 		_bank_lsb_spin.set_value (b & 127);
 	}