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 <peter.senna@gmail.com>
Signed-off-by: Jonathan Dieter <jdieter@lesbg.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/usb/usbip/src/usbip.c

@@ -176,6 +176,8 @@ int main(int argc, char *argv[])
 			break;
 		case '?':
 			printf("usbip: invalid option\n");
+			/* Terminate after printing error */
+			/* FALLTHRU */
 		default:
 			usbip_usage();
 			goto out;