externals: Track upstream libusb

We can place the external in an inner folder and manage the custom files
necessary to integrate it with CMake directly. This allows us to
directly change how we use it with our build system, as opposed to
needing to change a fork.

.gitmodules

@@ -16,6 +16,9 @@
 [submodule "libressl"]
     path = externals/libressl
     url = https://github.com/citra-emu/ext-libressl-portable.git
+[submodule "libusb"]
+	path = externals/libusb/libusb
+	url = https://github.com/libusb/libusb.git
 [submodule "discord-rpc"]
     path = externals/discord-rpc
     url = https://github.com/discordapp/discord-rpc.git