uapi: fix linux/seg6.h and linux/seg6_iptunnel.h userspace compilation errors

Include <linux/in6.h> in uapi/linux/seg6.h to fix the following
linux/seg6.h userspace compilation error:

/usr/include/linux/seg6.h:31:18: error: array type has incomplete element type 'struct in6_addr'
  struct in6_addr segments[0];

Include <linux/seg6.h> in uapi/linux/seg6_iptunnel.h to fix
the following linux/seg6_iptunnel.h userspace compilation error:

/usr/include/linux/seg6_iptunnel.h:26:21: error: array type has incomplete element type 'struct ipv6_sr_hdr'
  struct ipv6_sr_hdr srh[0];

Fixes: a50a05f4 ("ipv6: sr: add missing Kbuild export for header files")
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>