makefs: Fix a couple of HAVE_STRUCT_STAT_BIRTHTIME checks

These macros are meant to be 0/1 not undefined/1. Currently we don't
actually have nbtool_config.h included for these files so these are
always false, but that will change in a future commit and break building
on Linux where they are defined to 0.

Reviewed by: emaste, markj
Differential Revision: https://reviews.freebsd.org/D50077