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/libusb/strerror.c

@@ -34,7 +34,7 @@
 
 static size_t usbi_locale = 0;
 
-/** \ingroup misc
+/** \ingroup libusb_misc
  * How to add a new \ref libusb_strerror() translation:
  * <ol>
  * <li> Download the latest \c strerror.c from:<br>
@@ -125,7 +125,7 @@ static const char* usbi_localized_errors[ARRAYSIZE(usbi_locale_supported)][LIBUS
 	}
 };
 
-/** \ingroup misc
+/** \ingroup libusb_misc
  * Set the language, and only the language, not the encoding! used for
  * translatable libusb messages.
  *
@@ -176,7 +176,7 @@ int API_EXPORTED libusb_setlocale(const char *locale)
 	return LIBUSB_SUCCESS;
 }
 
-/** \ingroup misc
+/** \ingroup libusb_misc
  * Returns a constant string with a short description of the given error code,
  * this description is intended for displaying to the end user and will be in
  * the language set by libusb_setlocale().