lib:util: Fix parameter aliasing in tfork test

lib:util: Fix parameter aliasing in tfork test

../lib/util/tests/tfork.c:483:24: error: passing argument 1 to
    restrict-qualified parameter aliases with argument 4 [-Werror=restrict]
   ret = pthread_create(&threads[i],
                        ^~~~~~~~~~~
../lib/util/tests/tfork.c:486:10:
          (void *)&threads[i]);
          ~~~~~~~~~~~~~~~~~~~

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13437

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>