Correct spelling in ip_tunnels.h
As reported by codespell.

Cc: David Ahern <dsahern@kernel.org>
Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240822-net-spell-v1-4-3a98971ce2d2@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>