util/mkerr.pl: Add an inclusion of symhacks.h in all error files

This does no harm, and ensures that the inclusion isn't mistakenly
removed in the generated *err.h where it's actually needed.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8397)