lib: Use GLib and libusb directly for debug output

Use GLib internally to output debug information, and tell about
libusb's LIBUSB_DEBUG envvar for libusb debug.

fp_set_debug() is now a no-op.

https://bugs.freedesktop.org/show_bug.cgi?id=106552

libfprint/assembling.c

@@ -21,13 +21,14 @@
 
 #define FP_COMPONENT "assembling"
 
+#include "fp_internal.h"
+
 #include <errno.h>
 #include <string.h>
 
 #include <libusb.h>
 #include <glib.h>
 
-#include "fp_internal.h"
 #include "assembling.h"
 
 static unsigned int calc_error(struct fpi_frame_asmbl_ctx *ctx,