Fix warning:
```
serial/ptmx.c:205:34: warning: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 8 [-Wformat-truncation=]
205 | snprintf(devname, 16, "/dev/pty%d", minor);
|
serial/ptmx.c:205:25: note: directive argument in the range [0, 2147483647]
205 | snprintf(devname, 16, "/dev/pty%d", minor);
| ^~~~~~~~~~~~
serial/ptmx.c:205:3: note: ‘snprintf’ output between 10 and 19 bytes into a destination of size 16
205 | snprintf(devname, 16, "/dev/pty%d", minor);
```
Signed-off-by: Huang Qi <huangqi3@xiaomi.com>
d8a61108cf