Externals: Update libusb to version 1.0.21

1.0.21 adds a usbdk backend, which may be helpful as it could be
used in the future to work around the missing isochronous transfer
support for WinUSB backends.

Here is the full changelog (from libusb).

2016-10-01: v1.0.21:
* Core: Refactor code related to transfer flags and timeout handling
* Darwin: Ignore root hub simulation devices
* Darwin: Improved support for OS X El Capitan
* Darwin: Work around devices with buggy endpoint descriptors
* Darwin: Do not use objc_registerThreadWithCollector after its deprecation
* Darwin: Use C11 atomics on 10.12+ as the OS atomics are now deprecated
* Linux: Support preallocating kernel memory for zerocopy USB
* Linux: Deal with receiving POLLERR before all transfers have completed
* Solaris: Add solaris backend
* Windows: Add Visual Studio 2015 support
* Windows: Add usbdk backend
* Prevent attempts to recursively handle events
* Fix race condition in handle_timeout()
* Allow transferred argument to be optional in bulk APIs
* Various other bug fixes and improvements

Externals/libusb/README

@@ -1,15 +1,18 @@
-libusb
-======
+# libusb
+
+[![Build Status](https://travis-ci.org/libusb/libusb.svg?branch=master)](https://travis-ci.org/libusb/libusb)
+[![Build status](https://ci.appveyor.com/api/projects/status/xvrfam94jii4a6lw?svg=true)](https://ci.appveyor.com/project/LudovicRousseau/libusb)
+[![Coverity Scan Build Status](https://scan.coverity.com/projects/2180/badge.svg)](https://scan.coverity.com/projects/libusb-libusb)
 
 libusb is a library for USB device access from Linux, Mac OS X,
 Windows, OpenBSD/NetBSD and Haiku userspace.
 It is written in C (Haiku backend in C++) and licensed under the GNU
 Lesser General Public License version 2.1 or, at your option, any later
-version (see COPYING).
+version (see [COPYING](COPYING)).
 
 libusb is abstracted internally in such a way that it can hopefully
-be ported to other operating systems. Please see the PORTING file
-for more information.
+be ported to other operating systems. Please see the [PORTING](PORTING)
+file for more information.
 
 libusb homepage:
 http://libusb.info/
@@ -24,6 +27,7 @@ http://mailing-list.libusb.info
 - Hans de Goede <hdegoede@redhat.com>
 - Xiaofan Chen <xiaofanc@gmail.com>
 - Ludovic Rousseau <ludovic.rousseau@gmail.com>
-- Nathan Hjelm <hjelmn@users.sourceforge.net>
+- Nathan Hjelm <hjelmn@cs.unm.edu>
 - Chris Dickens <christopher.a.dickens@gmail.com>
+
 (Please use the mailing list rather than mailing developers directly)