openvswitch: Add Kconfig dependency on GRE-DEMUX.

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: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Pravin Shelar <pshelar@nicira.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>