magit.el: add `magit-diff-use-overlays' to `magit-faces' as custom-VARIABLE

Visiting the `magit-diff-use-overlays' option from the `magit-faces'
group resulted in an empty Customize buffer, stating:

  "Group definition missing."

See commit 66a06442.

Fixes #889.

Signed-off-by: Pieter Praet <pieter@praet.org>