Fixed some world breakage caused by not updating clients when <timeconv.h>

was split off from <time.h>.  This became fatal here when -Werror was
reenabled.

usr.sbin/ac/ac.c

@@ -29,6 +29,7 @@ static const char rcsid[] =
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <timeconv.h>
 #include <unistd.h>
 #include <utmp.h>
 

usr.sbin/lastlogin/lastlogin.c

@@ -42,6 +42,7 @@ __RCSID("$NetBSD: lastlogin.c,v 1.4 1998/02/03 04:45:35 perry Exp $");
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
+#include <timeconv.h>
 #include <utmp.h>
 #include <unistd.h>