datapath: Fix build failure on RHEL 6.4

datapath: Fix build failure on RHEL 6.4

Patch fixes following build failure:-

make[4]: Entering directory
`/usr/src/kernels/2.6.32-358.18.1.el6.x86_64'
  CC [M]  openvswitch/datapath/linux/actions.o
In file included from
openvswitch/datapath/linux/actions.c:21:
openvswitch/datapath/linux/compat/include/linux/skbuff.h:273:
error: redefinition of ‘__skb_fill_page_desc’
include/linux/skbuff.h:1123: note: previous definition of
‘__skb_fill_page_desc’ was here
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Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>