Fix warning: "pointer targets in passing argument 2 of...

Fix warning: "pointer targets in passing argument 2 of '_dbus_string_append_len' differ in signedness [-Wpointer-sign]".

Bug: https://bugs.freedesktop.org/attachment.cgi?id=93069Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

dbus/dbus-sha.c

@@ -460,7 +460,7 @@ _dbus_sha_final (DBusSHAContext   *context,
 
   sha_finish (context, digest);
 
-  if (!_dbus_string_append_len (results, digest, 20))
+  if (!_dbus_string_append_len (results, (const char *) digest, 20))
     return FALSE;
 
   /* some kind of security paranoia, though it seems pointless