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)