Flush stdout after writeln (fixes issue #13241)

Followup to pull https://github.com/D-Programming-Language/phobos/pull/2334

Fixes: https://issues.dlang.org/show_bug.cgi?id=13241

Not sure if the `.flush` call should be inside a version block for DMC, since apparently this isn't an issue on other platforms.
What I'd really like to know, is what DMC does differently from any other platform (e.g., Linux), where issue 13241 doesn't happen. This PR is really just a way to test out the waters in case the fix is relatively straightforward. Maybe somebody else should look into this, since I don't have a Win32 dev environment to troubleshoot with.