dist/tools: fix tunslip6 strncat num param

Cppcheck was (correctly) warning here that concat to strings might
result in buffer overflow because the terminating `\0` was not considered.
This is fixed here, making the cppcheck suppression also obsolete.

dist/tools/tunslip/tunslip6.c

@@ -651,7 +651,7 @@ devopen(const char *dev, int flags)
     char t[1024];
     strcpy(t, "/dev/");
     /* cppcheck-suppress bufferAccessOutOfBounds
-     * reason: seems to be a cppcheck bug */
+     * (reason: seems to be a bug in cppcheck 1.7x) */
     strncat(t, dev, sizeof(t) - 5);
     return open(t, flags);
 }