ip_tunnel: Move ip_tunnel_get_stats64 into ip_tunnel_core.c

net/built-in.o:(.rodata+0x1707c): undefined reference to `ip_tunnel_get_stats64'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>