qga: Remove unneeded include file

This file does not depend on windows.h.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>