Network Drop Monitor: Add trace declaration for skb frees

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>

 include/trace/skb.h   |    8 ++++++++
 net/core/Makefile     |    2 ++
 net/core/net-traces.c |   29 +++++++++++++++++++++++++++++
 3 files changed, 39 insertions(+)
Signed-off-by: David S. Miller <davem@davemloft.net>