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

sys/sys/elf_common.h

@@ -1,7 +1,7 @@
 /*-
  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
  *
- * Copyright (c) 2017 Dell EMC
+ * Copyright (c) 2017, 2018 Dell EMC
  * Copyright (c) 2000, 2001, 2008, 2011, David E. O'Brien
  * Copyright (c) 1998 John D. Polstra.
  * All rights reserved.
@@ -51,6 +51,7 @@ typedef struct {
 	u_int32_t	n_descsz;	/* Length of descriptor. */
 	u_int32_t	n_type;		/* Type of this note. */
 } Elf_Note;
+typedef Elf_Note Elf_Nhdr;
 
 /*
  * Option kinds.