Fix compilation using mingw.org's MinGW

[Eli Zaretskii]

> Cc: gdb-patches@sourceware.org
> From: Pedro Alves <palves@redhat.com>
> Date: Tue, 30 Apr 2019 17:31:22 +0100
> 
> On 4/30/19 4:23 PM, Eli Zaretskii wrote:
> >> Cc: gdb-patches@sourceware.org
> >> From: Pedro Alves <palves@redhat.com>
> >> Date: Tue, 30 Apr 2019 14:10:53 +0100
> >>
> >>>  - The override in _WIN32_WINNT is no longer necessary.  It could
> >>>    be removed at the same time.
> >>
> >> I meant the _WIN32_WINNT override in common/netstuff.c.
> >>
> >> I see now that there are more of those in other files too.
> > 
> > I will review them for the changeset on master, but I don't think we
> > should change them on the 8.3 branch.  OK?
> 
> Yes, my comments were for master, since changing removing the overrides
> in the other files only makes sense with the _WIN32_WINNT definition
> in common-defs.h.

OK, done.  I think I removed all the overrides that were there.

Thanks.