Remove a bogus cast and lockdown users(1) with WARNS?=2

Submitted by:	David Hill <david@phobia.ms>
Reviewed by:	-audit
MFC after:	1 week

usr.bin/users/Makefile

@@ -1,5 +1,7 @@
 #	@(#)Makefile	8.1 (Berkeley) 6/6/93
+# $FreeBSD$
 
 PROG=	users
+WARNS?=	2
 
 .include <bsd.prog.mk>

usr.bin/users/users.c

@@ -117,5 +117,5 @@ int
 scmp(p, q)
 	const void *p, *q;
 {
-	return(strncmp((char *)p, (char *)q, UT_NAMESIZE));
+	return(strncmp(p, q, UT_NAMESIZE));
 }