Fix make makeman after r367577

WITH_INIT_ALL_ZERO and WITH_INIT_ALL_PATTERN are mutually exclusive.
The .error when they were both set broke makeman so demote it to a
warning (and presumably the compiler will fail on an error later on).

We could improve this to make one take precedence but this is sufficient
for now.

MFC with: r367577
Sponsored by: The FreeBSD Foundation