LSM: lsm_hooks.h - fix missing colon in docstring

Apparently without it it is incorrect syntax and causes a warning about
undocumented struct field.

Fixes: b230d5ab

 ("LSM: add new hook for kernfs node initialization")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>