libusb-1.0: Fix the LibUSB.HotPlugCb declaration

It returns an int used to decide whether the callback should be rearmed
or deregistered.

Co-authored-by: Håvard Sørbø <havard@hsorbo.no>

vapi/libusb-1.0.vapi

@@ -567,7 +567,7 @@ namespace LibUSB {
 	public delegate void pollfd_removed_cb (int fd, void* user_data);
 
 	[CCode (cname = "libusb_hotplug_callback_fn")]
-	public delegate void HotPlugCb (Context ctx, Device device, HotPlugEvent event);
+	public delegate int HotPlugCb (Context ctx, Device device, HotPlugEvent event);
 
 	[CCode (cname = "struct libusb_pollfd")]
 	[Compact]