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>

pretty.c

@@ -1584,8 +1584,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	case 'D':
 		{
 			const struct decoration_options opts = {
-				.prefix = "",
-				.suffix = ""
+				.prefix = (char *) "",
+				.suffix = (char *) "",
 			};
 
 			format_decorations(sb, commit, c->auto_color, &opts);