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>

net/openvswitch/Makefile

@@ -10,10 +10,13 @@ openvswitch-y := \
 	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

net/openvswitch/vport-gre.c

@@ -16,7 +16,6 @@
  * 02110-1301, USA
  */
 
-#ifdef CONFIG_OPENVSWITCH_GRE
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/if.h>
@@ -271,5 +270,3 @@ const struct vport_ops ovs_gre_vport_ops = {
 	.get_name	= gre_get_name,
 	.send		= gre_tnl_send,
 };
-
-#endif /* OPENVSWITCH_GRE */