Fix compilation with older MinGW headers

Headers from mingw.org (4.7.2) declares _flushall only if __STRICT_ANSI__
is not defined ... Anyhow, we can as well use fflush(), which is in the
official standard.

Change-Id: Ic2b4d2ac724280f1304221be1fceab067af0c1f8
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -954,7 +954,7 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
         qGuiAppPriv->commitData();
 
         if (lParam & ENDSESSION_LOGOFF)
-            _flushall();
+            fflush(NULL);
 
         return !sessionManager->wasCanceled();
     }