openvswitch: fix linking without CONFIG_NF_CONNTRACK_LABELS

When CONFIG_CC_OPTIMIZE_FOR_DEBUGGING is enabled, the compiler
fails to optimize out a dead code path, which leads to a link failure:

net/openvswitch/conntrack.o: In function `ovs_ct_set_labels':
conntrack.c:(.text+0x2e60): undefined reference to `nf_connlabels_replace'

In this configuration, we can take a shortcut, and completely
remove the contrack label code. This may also help the regular
optimization.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>