Include <nlist.h> for nlist interfaces instead of depending on namespace

pollution in <kvm.h>.

usr.sbin/iostat/iostat.c

@@ -109,6 +109,7 @@
 #include <ctype.h>
 #include <fcntl.h>
 #include <kvm.h>
+#include <nlist.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>