libX11: fix overflow warning?

```
imLcLkup.c: In function ‘_XimLocalUtf8LookupString’:
imLcLkup.c:213:6: warning: overflow in implicit constant conversion [-Woverflow]
      buffer[0] = 0xe0 | ((BRL_UC_ROW >> 12) & 0x0f);
```

Currently it is unclear how to fix this.
