Openvswitch uses function from NET_IPGRE_DEMUX module.
Add Kconfig dependency to fix following compilation errors:
http://marc.info/?l=linux-netdev&m=137244035226634

CC: Jesse Gross <jesse@nicira.com>
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NPravin Shelar <pshelar@nicira.com>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Acked-by: NJesse Gross <jesse@nicira.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This bug was introduced by commit aa310701
(openvswitch: Add gre tunnel support.)
Signed-off-by: NPravin B Shelar <pshelar@nicira.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Add gre vport implementation.  Most of gre protocol processing
is pushed to gre module. It make use of gre demultiplexer
therefore it can co-exist with linux device based gre tunnels.
Signed-off-by: NPravin B Shelar <pshelar@nicira.com>
Acked-by: NJesse Gross <jesse@nicira.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>