[NET]: Only build flow.o if CONFIG_XFRM=y

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/Makefile

@@ -7,9 +7,10 @@ obj-y := sock.o request_sock.o skbuff.o iovec.o datagram.o stream.o scm.o \
 
 obj-$(CONFIG_SYSCTL) += sysctl_net_core.o
 
-obj-y		     += flow.o dev.o ethtool.o dev_mcast.o dst.o \
+obj-y		     += dev.o ethtool.o dev_mcast.o dst.o \
 			neighbour.o rtnetlink.o utils.o link_watch.o filter.o
 
+obj-$(CONFIG_XFRM) += flow.o
 obj-$(CONFIG_SYSFS) += net-sysfs.o
 obj-$(CONFIG_NETFILTER) += netfilter.o
 obj-$(CONFIG_NET_DIVERT) += dv.o