Fix wrong buffer size

Fix build issue:

```
/tmp/libfreefare/libfreefare/ntag21x.c:718:5: error: array index 1 is past the end of the array
      (which contains 1 element) [-Werror,-Warray-bounds]
    cmd_step1[1] = 0x00;
    ^         ~
/tmp/libfreefare/libfreefare/ntag21x.c:715:5: note: array 'cmd_step1' declared here
    uint8_t cmd_step1[1];
    ^
1 error generated.
```

libfreefare/ntag21x.c

@@ -712,7 +712,7 @@ bool
 ntag21x_is_auth_supported(nfc_device *device, nfc_iso14443a_info nai)
 {
     int ret;
-    uint8_t cmd_step1[1];
+    uint8_t cmd_step1[2];
     uint8_t res_step1[8];
     cmd_step1[0] = 0x60;
     cmd_step1[1] = 0x00;