Configure: Improve warning if no random seed source was configured

The new Configure summary box (41349b5e) now hides the warning
about the missing seed source (2805ee1e

) too much. To make it
more visible again, add warning markers.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7509)

(cherry picked from commit caa85952)