Add Elf_Nhdr definition to match NetBSD, OpenBSD, Linux

The mesa port started to use this type and fails to build without it.

NetBSD: http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/sys/exec_elf.h.diff?r1=1.26&r2=1.27&f=h
OpenBSD: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/sys/sys/exec_elf.h.diff?r1=1.21&r2=1.22&f=h

PR: 225302
Reported by: Greg V <greg AT unrelenting.technology>
Sponsored by: Dell EMC Isilon