net/core: make function ___gnet_stats_copy_basic() static

Fixes the following sparse warning:

net/core/gen_stats.c:166:1: warning:
 symbol '___gnet_stats_copy_basic' was not declared. Should it be static?

Fixes: 5e111210 ("net/core: Add new basic hardware counter")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>