bpfilter: fix a build err

gcc-7.3.0 report following err:

  HOSTCC  net/bpfilter/main.o
In file included from net/bpfilter/main.c:9:0:
./include/uapi/linux/bpf.h:12:10: fatal error: linux/bpf_common.h: No such file or directory
 #include <linux/bpf_common.h>

remove it by adding a include path.
Fixes: d2ba09c1

 ("net: add skeleton of bpfilter kernel module")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>