net: hns3: fix compile error

Currently, the rules for configuring search paths in Kbuild have
changed, this will lead some erros when compiling hns3 with the
following command:

make O=DIR M=drivers/net/ethernet/hisilicon/hns3

drivers/net/ethernet/hisilicon/hns3/hns3pf/hclge_cmd.c:11:10:
fatal error: hnae3.h: No such file or directory

This patch fix it by adding $(srctree)/ prefix to the serach paths.

Signed-off-by: Xi Wang <wangxi11@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>