openvswitch: check CONFIG_OPENVSWITCH_GRE in makefile

Cc: Jesse Gross <jesse@nicira.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
diff --git a/net/openvswitch/Makefile b/net/openvswitch/Makefile
index 82e4ee5..ea36e99 100644
--- a/net/openvswitch/Makefile
+++ b/net/openvswitch/Makefile
@@ -10,10 +10,13 @@
 	dp_notify.o \
 	flow.o \
 	vport.o \
-	vport-gre.o \
 	vport-internal_dev.o \
 	vport-netdev.o
 
 ifneq ($(CONFIG_OPENVSWITCH_VXLAN),)
 openvswitch-y += vport-vxlan.o
 endif
+
+ifneq ($(CONFIG_OPENVSWITCH_GRE),)
+openvswitch-y += vport-gre.o
+endif