Fix buffer overrun crash running MSVC2010 win32 Qt on win64

Fix buffer overrun crash running MSVC2010 win32 Qt on win64

When loading dnsapi.dll as a plugin, we experience crashes because
the calling convention is not specified.
The default is _cdecl, but __stdcall (via the WINAPI macro) should
be used for windows APIs.
Mismatched calling convention results in corruption of local variables,
probably because the stack pointer is incorrect and SP offsets are
used in optimised builds rather than frame pointer offsets.

Since the library has been available since Windows 2000, I don't
think that we need to load it dynamically.
(Unlike the unix version where it isn't part of the LSB)

Also checked that the current release of mingw works.

Task-number: QTBUG-24227
Change-Id: I37c0a6aa0c133799c2a6dd9391ca1435ba2539ea
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>