Externals: update libusb to version 1.0.19

Externals/libusb/README

@@ -0,0 +1,27 @@
+libusb
+======
+
+libusb is a library for USB device access from Linux, Mac OS X,
+Windows and OpenBSD/NetBSD userspace.
+It is written in C and licensed under the GNU Lesser General Public
+License version 2.1 or, at your option, any later version (see 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.
+
+libusb homepage:
+http://libusb.info/
+
+Developers will wish to consult the API documentation:
+http://api.libusb.info
+
+Use the mailing list for questions, comments, etc:
+http://mailing-list.libusb.info
+
+- Pete Batard <pete@akeo.ie>
+- Hans de Goede <hdegoede@redhat.com>
+- Xiaofan Chen <xiaofanc@gmail.com>
+- Ludovic Rousseau <ludovic.rousseau@gmail.com>
+- Nathan Hjelm <hjelmn@users.sourceforge.net>
+(Please use the mailing list rather than mailing developers directly)