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>
diff --git a/net/openvswitch/Kconfig b/net/openvswitch/Kconfig
index 9fbc04a..27ee56b 100644
--- a/net/openvswitch/Kconfig
+++ b/net/openvswitch/Kconfig
@@ -19,8 +19,6 @@
 	  which is able to accept configuration from a variety of sources and
 	  translate it into packet processing rules.
 
-	  Open vSwitch GRE support depends on CONFIG_NET_IPGRE_DEMUX.
-
 	  See http://openvswitch.org for more information and userspace
 	  utilities.
 
@@ -28,3 +26,17 @@
 	  called openvswitch.
 
 	  If unsure, say N.
+
+config OPENVSWITCH_GRE
+	bool "Open vSwitch GRE tunneling support"
+	depends on INET
+	depends on OPENVSWITCH
+	depends on NET_IPGRE_DEMUX && !(OPENVSWITCH=y && NET_IPGRE_DEMUX=m)
+	default y
+	---help---
+	  If you say Y here, then the Open vSwitch will be able create GRE
+	  vport.
+
+	  Say N to exclude this support and reduce the binary size.
+
+	  If unsure, say Y.