usb: Support compilation without poll.h

This is a hack to support compilation with Mingw-w64 which provides
a libusb-1.0 package, but no poll.h.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1458630800-10088-1-git-send-email-sw@weilnetz.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
