net: Include private header in more needed places

sys/netinet and sys/netipsec are both part of the 'blessed' netstack, so
can access struct ifnet directly.  With this structure becoming private
very soon, the necessary files need to get direct access.

Sponsored by:	Juniper Networks, Inc.

sys/netinet/tcp_lro_hpts.c

@@ -39,6 +39,7 @@
 
 #include <net/if.h>
 #include <net/if_var.h>
+#include <net/if_private.h>
 #include <net/ethernet.h>
 #include <net/bpf.h>
 #include <net/vnet.h>

sys/netipsec/ipsec_offload.c

@@ -44,6 +44,7 @@
 
 #include <net/if.h>
 #include <net/if_var.h>
+#include <net/if_private.h>
 #include <net/vnet.h>
 #include <netinet/in.h>
 #include <netinet/ip.h>