message: Restore explicit flushing of messages on stderr

In the `cmake` command-line tool, the `message()` command with no
message mode argument prints the message stderr using the C++ `cerr`
stream.  Since commit 0a0a0f8a (cmMessenger: Color messages to
terminal by type, 2021-05-18, v3.21.0-rc1~146^2) and an update by
commit c7a8c9c8 (cmMessenger: Revert to non-color messages on
Windows, 2021-07-20, v3.21.1~15^2), we print the newline at the end of
the message using just `\n`.  We've now observed some cases of output
on stdout and stderr getting jumbled when the two go to the same file
descriptor.  Previously the newline was printed with `endl`, which
implicitly flushes.  Flush explicitly to restore that behavior.

Fixes: #23155