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.
Would you like to take over this PR? I don't have a Windows dev environment (much less a Windows DMC dev environment) so I have no way of locally testing this fix at all. I'd just be shooting in the dark and hoping it works (and we don't even have a unittest capable of testing this, so I wouldn't even know if the fix actually worked or not).