build-sys: don't include windows.h, osdep.h does it

osdep.h will also define the available Windows API version for QEMU.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181122110039.15972-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>