Include the <sys/mman.h> header for NetBSD mkerrors.sh. This brings
in constants used with mmap(2), msync(2) and mlockall(2).

The regeneration of the NetBSD zerror* files also picks clone(2)
related constants.

Update #4929.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/45510044