xfce4-hdaps-dialogs.c: replace obsolete gtk_misc_set_alignment().

The gtk_misc_set_alignment() function has been deprecated since
gtk+-3.14. Instead, we are prodded to use the gtk_widget_set_halign()
and gtk_widget_set_valign() functions. This is a trivial change,
replacing two calls to gtk_misc_set_alignment() by four calls to
the new functions.