usbip: Fix implicit fallthrough warning

GCC 7 now warns when switch statements fall through implicitly, and with
-Werror enabled in configure.ac, that makes these tools unbuildable.

We fix this by notifying the compiler that this particular case statement
is meant to fall through.

Reviewed-by: Peter Senna Tschudin <[e-mail]>
Signed-off-by: Jonathan Dieter <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

tools/usb/usbip/src/usbip.c

···
 			break;
 		case '?':
 			printf("usbip: invalid option\n");
+			/* Terminate after printing error */
+			/* FALLTHRU */
 		default:
 			usbip_usage();
 			goto out;