usbip: Fix implicit fallthrough warning

commit cfd6ed4537a9e938fa76facecd4b9cd65b6d1563 upstream.

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: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/tools/usb/usbip/src/usbip.c b/tools/usb/usbip/src/usbip.c
index d7599d9..73d8eee 100644
--- a/tools/usb/usbip/src/usbip.c
+++ b/tools/usb/usbip/src/usbip.c
@@ -176,6 +176,8 @@
 			break;
 		case '?':
 			printf("usbip: invalid option\n");
+			/* Terminate after printing error */
+			/* FALLTHRU */
 		default:
 			usbip_usage();
 			goto out;