dummymbuf: Explicitly include if_private.h

struct ifnet is expected to be private for everything outside of the
network stack (sys/net*, except netlink.  Since dummymbuf is part of the
network stack, explicitly include the header to get access to the
private members.

Sponsored by:	Juniper Networks, Inc.

sys/net/dummymbuf.c

@@ -37,6 +37,7 @@
 
 #include <net/if.h>
 #include <net/if_var.h>
+#include <net/if_private.h>
 #include <net/vnet.h>
 #include <net/pfil.h>