Add missing header.

S_IRUSR is defined in sys/stat.h

PR:		209229
Submitted by:	<mt AT markoturk DOT info>
Approved by:	bcr (mentor)
MFC after:	5 days
Differential Revision:	https://reviews.freebsd.org/D12007

lib/libc/sys/shmget.2

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 17, 2010
+.Dd August 12, 2017
 .Dt SHMGET 2
 .Os
 .Sh NAME
@@ -35,6 +35,7 @@
 .Lb libc
 .Sh SYNOPSIS
 .In sys/types.h
+.In sys/stat.h
 .In sys/ipc.h
 .In sys/shm.h
 .Ft int