Flush stderr on Windows
Flush stderr on Windows

While on Linux stderr is unbuffered, it is buffered on Windows. Always
flush stderr on Windows to ensure any error messages appear in the right
order to ease diagnosing.

Change-Id: I37300e384ecd3a51a321a48818f0114d6f3357a0

TheQtCompany/git-repo,couchbasedeps/git-repo,mer-tools/git-repo,GerritCodeReview/git-repo,aosp-mirror/tools_repo,lipro-yocto/git-repo,couchbasedeps/git-repo,codetutil/git-repo,gabbayo/git-repo,lewixliu/git-repo,GerritCodeReview/git-repo,mer-tools/git-repo,chenzilin/git-repo,lipro-yocto/git-repo,lewixliu/git-repo,chenzilin/git-repo,TheQtCompany/git-repo,gabbayo/git-repo,codetutil/git-repo,aosp-mirror/tools_repo