netfilter: nf_conntrack: fix linker error with NF_CONNTRACK_TIMESTAMP=n

net/built-in.o: In function `nf_conntrack_init_net':
net/netfilter/nf_conntrack_core.c:1521:
	undefined reference to `nf_conntrack_tstamp_init'
net/netfilter/nf_conntrack_core.c:1531:
	undefined reference to `nf_conntrack_tstamp_fini'

Add dummy inline functions for the =n case to fix this.
Reported-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/net/netfilter/nf_conntrack_timestamp.h

@@ -47,7 +47,19 @@ static inline void nf_ct_set_tstamp(struct net *net, bool enable)
 	net->ct.sysctl_tstamp = enable;
 }
 
+#ifdef CONFIG_NF_CONNTRACK_TIMESTAMP
 extern int nf_conntrack_tstamp_init(struct net *net);
 extern void nf_conntrack_tstamp_fini(struct net *net);
+#else
+static inline int nf_conntrack_tstamp_init(struct net *net)
+{
+	return 0;
+}
+
+static inline void nf_conntrack_tstamp_fini(struct net *net)
+{
+	return;
+}
+#endif /* CONFIG_NF_CONNTRACK_TIMESTAMP */
 
 #endif /* _NF_CONNTRACK_TSTAMP_H */