Specify signed char instead of char

I got this compilation warning when compiling for aarch64

```
yuv_io.c: In function 'read_frame_header':
yuv_io.c:210:21: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  210 |       if (buffer[i] == EOF) return 0;
      |                     ^~
```

Since char is unsigned on arm, should we specify `signed char` in general?