ipv6: sr: add missing Kbuild export for header files

Add missing IPv6-SR header files in include/uapi/linux/Kbuild.

Also, prevent seg6_lwt_headroom() from being exported and add
missing linux/types.h include.

Signed-off-by: David Lebrun <david.lebrun@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>