fix spin button in preferences

This fixes the appearance of the "Pan amplitude" spin button in the xmp plugin preferences. The fix has been tested with Audacious 3.4, 3.4.1 and 3.4.2.

It is not known to me [yet] when this has been broken, but the two empty "" strings (for the labels at the left and the right of the button) have been the culprit. In the options dialog, they have been replaced with the full header of the translated message object file looking obviously broken. ;)