ubihealthd: Add missing sentinel in options array

`getopt_long()` requires a null terminated array, otherwise we get
segfaults when passing invalid options.

Fixes: 7f0e2dc2 ("ubi-utils: Implement a ubihealthd")
Signed-off-by: Alexander Dahl <post@lespocky.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>