Disable -Wcast-function-type-mismatch

Code owners should fix this.
```
...ternal/musl/src/thread/thrd_create.c:6:54: error: cast from 'thrd_start_t' (aka 'int (*)(void *)') to 'void *(*)(void *)' converts to incompatible function type [-Werror,-Wcast-function-type-mismatch]
```
Bug: b/353528991
Test: m checkbuild

Change-Id: Ie5a670d5714b4c439a43cac96f93c642ef817103