psample: Add a fwd declaration for skbuff

Without this there is a warning if source files include psample.h
before skbuff.h or doesn't include it at all.

Fixes: 6ae0a628 ("net: Introduce psample, a new genetlink channel for packet sampling")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Link: https://lore.kernel.org/r/20210808065242.1522535-1-roid@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>