tools: image-host.c: use correct variable for strerrno

In the function get_random_data, strerrno is called with
the variable ret (which is the return of the function
clock_gettime). It should be called with errnor. This
commit fixes this mistake.

Reported-by: Coverity (CID: 312956)
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>