net: Add dependent headers to trace/skb.h

The tracing header needs to include definitions for the macros used and
the types referenced.  This lets automated tracing tools like SystemTap
make use of the tracepoint without any specific knowledge of its
meaning (leaving that to the user).

Signed-off-by: Josh Stone <jistone@redhat.com>
CC: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>