Update the sysctl(9) manpage to indicate that <sys/param.h> is required

instead of <sys/types.h>.  (<sys/sysctl.h> includes NULL, which is defined
with <sys/param.h> and not <sys/types.h>.)

Sponsored by:	Netflix

share/man/man9/sysctl.9

@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 3, 2018
+.Dd June 1, 2018
 .Dt SYSCTL 9
 .Os
 .Sh NAME
@@ -80,7 +80,7 @@
 .Nm SYSCTL_UQUAD
 .Nd Dynamic and static sysctl MIB creation functions
 .Sh SYNOPSIS
-.In sys/types.h
+.In sys/param.h
 .In sys/sysctl.h
 .Fn SYSCTL_DECL name
 .Ft struct sysctl_oid *