unix: fix mkerrors.sh on NetBSD

[tbvdm]

Running mkall.sh on NetBSD/amd64 9.3 fails:

$ GOOS=netbsd GOARCH=amd64 ./mkall.sh
/home/tim/sys/unix/_const.go:249:15: could not determine kind of name for C.DIOCAWEDGE
/home/tim/sys/unix/_const.go:251:15: could not determine kind of name for C.DIOCBSLIST
/home/tim/sys/unix/_const.go:254:15: could not determine kind of name for C.DIOCDWEDGE
/home/tim/sys/unix/_const.go:257:18: could not determine kind of name for C.DIOCGDEFLABEL
/home/tim/sys/unix/_const.go:258:15: could not determine kind of name for C.DIOCGDINFO
/home/tim/sys/unix/_const.go:261:21: could not determine kind of name for C.DIOCGSECTORALIGN
/home/tim/sys/unix/_const.go:263:18: could not determine kind of name for C.DIOCGSTRATEGY
/home/tim/sys/unix/_const.go:264:19: could not determine kind of name for C.DIOCGWEDGEINFO
/home/tim/sys/unix/_const.go:267:16: could not determine kind of name for C.DIOCLWEDGES
/home/tim/sys/unix/_const.go:269:16: could not determine kind of name for C.DIOCRFORMAT
/home/tim/sys/unix/_const.go:271:13: could not determine kind of name for C.DIOCSBAD
/home/tim/sys/unix/_const.go:273:15: could not determine kind of name for C.DIOCSDINFO
/home/tim/sys/unix/_const.go:276:18: could not determine kind of name for C.DIOCSSTRATEGY
/home/tim/sys/unix/_const.go:278:15: could not determine kind of name for C.DIOCWDINFO
/home/tim/sys/unix/_const.go:279:16: could not determine kind of name for C.DIOCWFORMAT

(I tried with Go 1.18.3 from NetBSD's package manager and with Go 1.20.2 built from source.)

The reported names are ioctls defined in <sys/dkio.h>. These ioctls refer to types defined in headers that apparently are not included. This PR adds these headers to mkerr.sh, but it does not include the changes in the regenerated files.

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[tbvdm]

I haven't signed a CLA. Please feel free to commit this trivial change yourself.