net_sched: do not export gnet_stats_basic_packed to uapi

gnet_stats_basic_packed was really meant to be private kernel structure.

If this proves to be a problem, we will have to rename the in-kernel
version.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>