SUNRPC: Remove "#include <trace/events/skb.h>"

Clean up: Commit 850cbadd

 ("udp: use it's own memory accounting
schema") removed the last skb-related tracepoint from svcsock.c, so
it is no longer necessary to include trace/events/skb.h.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>