build failure for amd64

[Paul Goyette <paul%whooppee.com@localhost>]

With sources updated on Wed Jul  1 14:05:15 UTC 2020, and with a
completely empty $OBJDIR and $DESTDIR, I got the following error:

...
depend ===> etc/amd64/kmod
nbmake[4]: nbmake[4]: don't know how to make miniroot.kmod.c. Stop
nbmake[4]: stopped in $SRCDIR/distrib/amd64/kmod


+--------------------+--------------------------+-----------------------+
| Paul Goyette       | PGP Key fingerprint:     | E-mail addresses:     |
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