Include structure changed in Visual Studio 16.6.1

[Yu Jin]

I have recently updated Visual Studio to 16.6.1 and could not compile
support anymore. I investigated this and it seems that before 16.6.1 "endl"
(os_win32.cpp: 178) was declared through following include structure:
QString > qstring.h > iterator > istream > ostream
In 16.6.1 istream is replaced with iosfwd in iterator and "endl" is no
longer declared.
Please commit the attached patch to fix this.
It is also possible to use iostream instead of ostream as done in Stable
[092bc93903f7101b322e3023238c8fc011560273/lyxgit]
<https://www.lyx.org/trac/changeset/092bc93903f7101b322e3023238c8fc011560273/lyxgit>

Eugene
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