[dev] [st] Fix terminfo op for light backgrounds

Hi,

Attached is a tiny change to st's terminfo entry to set 'op' (original
pair) to \E[39;49m. This is akin to what urxvt and xterm do, and fixes
some issues in certain curses apps (e.g. top) if your terminal had a
color scheme other than white on black.

Cheers,
Rafa.