seg6_iptunnel: Refactor seg6_lwt_headroom out of uapi header

Refactor the function seg6_lwt_headroom out of the seg6_iptunnel.h uapi
header, because it is only used in seg6_iptunnel.c. Moreover, it is only
used in the kernel code, as indicated by the "#ifdef __KERNEL__".

Suggested-by: David Miller <davem@davemloft.net>
Signed-off-by: Ioana-Ruxandra Stăncioi <stancioi@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>