libusb-sniff: avoid usb.h include

usb.h is only required for the definition of usb_dev_handle,
which is never used, so it can be defined as anonymous struct
instead. This has the advantage, that libusb is not required
on the system compiling the library.

src/libusb-sniff.c

@@ -25,7 +25,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <dlfcn.h>
-#include <usb.h>
+
+struct usb_dev_handle;
+typedef struct usb_dev_handle usb_dev_handle;
 
 static char to_ascii(char c) {