datapath: Fix compilation on kernel 2.6.32

datapath: Fix compilation on kernel 2.6.32

Fixes following compilation error:

CC [M]  /home/travis/build/openvswitch/ovs/datapath/linux/actions.o

In file included from
/home/travis/build/openvswitch/ovs/datapath/linux/actions.c:21:0:

/home/travis/build/openvswitch/ovs/datapath/linux/compat/include/linux/skbuff.h:
In function ‘rpl_skb_postpull_rcsum’:

/home/travis/build/openvswitch/ovs/datapath/linux/compat/include/linux/skbuff.h:384:4:
error: implicit declaration of function ‘skb_checksum_start_offset’
[-Werror=implicit-function-declaration]

cc1: some warnings being treated as errors

Reported-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>