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

include/net/psample.h

@@ -31,6 +31,8 @@ struct psample_group *psample_group_get(struct net *net, u32 group_num);
 void psample_group_take(struct psample_group *group);
 void psample_group_put(struct psample_group *group);
 
+struct sk_buff;
+
 #if IS_ENABLED(CONFIG_PSAMPLE)
 
 void psample_sample_packet(struct psample_group *group, struct sk_buff *skb,