xfrm: Correct spelling mistake in xfrm.h comment

A spelling error was found in the comment section of
include/uapi/linux/xfrm.h. Since this header file is copied to many
userspace programs and undergoes Debian spellcheck, it's preferable to
fix it in upstream rather than downstream having exceptions.

This commit fixes the spelling mistake.

Fixes: df71837d ("[LSM-IPSec]: Security association restriction.")
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>