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>