kdd: fix build again

Restore Tim's patch. The one that was committed was recreated by me
because git didn't accept my saved copy. I made some mistakes while
recreating that patch and here we are.

Fixes: 3471caf ("kdd: stop using [0] arrays to access packet contents")
Reported-by: Michael Young <m.a.young@durham.ac.uk>
Signed-off-by: Wei Liu <wl@xen.org>
Reviewed-by: Tim Deegan <tim@xen.org>
Release-acked-by: Paul Durrant <paul@xen.org>
(cherry picked from commit d44cbbe)

tools/debugger/kdd/kdd.c

@@ -67,11 +67,11 @@ typedef struct {
 /* State of the debugger stub */
 typedef struct {
     union {
-        uint8_t txb[sizeof (kdd_hdr)];           /* Marshalling area for tx */
+        uint8_t txb[sizeof (kdd_pkt)];           /* Marshalling area for tx */
         kdd_pkt txp;                 /* Also readable as a packet structure */
     };
     union {
-        uint8_t rxb[sizeof (kdd_hdr)];           /* Marshalling area for rx */
+        uint8_t rxb[sizeof (kdd_pkt)];           /* Marshalling area for rx */
         kdd_pkt rxp;                 /* Also readable as a packet structure */
     };
     unsigned int cur;       /* Offset into rx where we'll put the next byte */