Restore s:bg_base02 on SignColumn

For some reason, SignColumn lost its background in commit 295f56b0121a,
without even s:bg_none, which made it an ugly out-of-scheme gray color.
It was the only highlight group without any bg setting.  It's now back
to s:bg_base02, which is the same as LineNr that's also in the gutter.

For instance, this showed up in vim-signify's unmarked lines, whereas
its marked lines use Diff* fg/bg colors.