pretty: add casts for decoration option pointers

pretty: add casts for decoration option pointers

The `struct decoration_options` have a prefix and suffix field which are
both non-constant, but we assign a constant pointer to them. This is
safe to do because we pass them to `format_decorations()`, which never
modifies these pointers, and then immediately discard the structure. Add
explicit casts to avoid compilation warnings with `-Wwrite-strings`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>