Compatibility with 4.5 Kernel

[Salz]

Compiling the host modules fails if CONFIG_NET_CLS_ACT is set with:

  CC [M]  /var/lib/dkms/virtualbox/5.0.18/build/vboxnetflt/linux/VBoxNetFlt-linux.o
In file included from include/linux/filter.h:16:0,
                 from include/net/sock.h:64,
                 from include/linux/tcp.h:22,
                 from include/linux/ipv6.h:72,
                 from include/net/ipv6.h:16,
                 from /var/lib/dkms/virtualbox/5.0.18/build/vboxnetflt/linux/VBoxNetFlt-linux.c:38:
include/net/sch_generic.h: In function ‘skb_at_tc_ingress’:
include/net/sch_generic.h:413:9: error: implicit declaration of function ‘G_TC_AT’ [-Werror=implicit-function-declaration]
  return G_TC_AT(skb->tc_verd) & AT_INGRESS;
         ^
include/net/sch_generic.h:413:33: error: ‘AT_INGRESS’ undeclared (first use in this function)
  return G_TC_AT(skb->tc_verd) & AT_INGRESS;
                                 ^
include/net/sch_generic.h:413:33: note: each undeclared identifier is reported only once for each function it appears in
cc1: some warnings being treated as errors
scripts/Makefile.build:258: recipe for target '/var/lib/dkms/virtualbox/5.0.18/build/vboxnetflt/linux/VBoxNetFlt-linux.o' failed

This is due to a change in the kernel headers in 4.5: ​https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=fdc5432

The appended patch fixes compilation on linux 4.5