mango (mangowc) scroller: gappoh ignored with 2+ windows

In mangowc's scroller layout, the outer horizontal gap gappoh only applies when a single window is on the tag. With two or more columns, the side margin comes from scroller_structs (src/layout/scroll.c computes max width as monitor width - 2*scroller_structs - gappih). So scroller_structs=0 makes multi-window layouts touch the screen edges even with gappoh=10. Set scroller_structs to the same value as gappoh to keep the edge gap consistent, e.g. so windows line up with a floating bar's margin.