Constify libusb_get_pollfds return

The correct return type, per our libusb(3) man page and the libusb.org
and libusbx projects (whose interface we intend to follow for our libusb),
is const struct libusb_pollfd **.

Unfortunately the doxygen-generated libusbx API documentation[1] omits[2]
the const for some reason.

[1] http://libusbx.sourceforge.net/api-1.0/group__poll.html#gab1a72869a926552b27a6c667695df3a2

[2] http://sourceforge.net/mailarchive/forum.php?thread_name=497D10BE.8090007%40n-dimensional.de&forum_name=libusb-devel

Reviewed by: hselasky@