vlan_mon: make vlan_mon.h self-contained

We need to include <stdint.h> to define 'uint16_t'.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>

accel-pppd/include/vlan_mon.h

 #ifndef __VLAN_MON_H
 #define __VLAN_MON_H
 
+#include <stdint.h>
+
 typedef void (*vlan_mon_notify)(int ifindex, int vid, int vlan_ifindex);
 
 void vlan_mon_register_proto(uint16_t proto, vlan_mon_notify cb);