Use gousb_set_debug instead of libusb_set_debug in one more place. (#44)

* One more place that should call gousb_set_debug instead of
libusb_set_debug.

* Better comment to explain what sort of benchmarking this is needed for.

libusb.go

@@ -498,9 +498,9 @@ func xferCallback(xfer *C.struct_libusb_transfer) {
 	ch <- struct{}{}
 }
 
-// for benchmarking and testing
+// for benchmarking of method on implementation vs vanilla function.
 func libusbSetDebug(c *libusbContext, lvl int) {
-	C.libusb_set_debug((*C.libusb_context)(c), C.int(lvl))
+	C.gousb_set_debug((*C.libusb_context)(c), C.int(lvl))
 }
 
 func newDevicePointer() *libusbDevice {