tun: Small changes I needed to get criu working under 3.18.1 kernel on Centos 6.5 machine.

Signed-off-by: Mark A. O'Neill <mao@tumblingdice.co.uk>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

tun.c

@@ -4,6 +4,9 @@
 #include <linux/if_tun.h>
 #include <sys/ioctl.h>
 
+// MAO required on Centos 6 (linux-3.18.1 kernel)
+#include <linux/filter.h>
+
 #include "cr_options.h"
 #include "imgset.h"
 #include "protobuf.h"