libcephfs_proxy: remove arithmetic on void*

libcephfs_proxy: remove arithmetic on void*

this change is created in the same spirit of bb1fa818.

when building the tree with clang-21, following warning was raised:
```
/home/kefu/dev/ceph/src/libcephfs_proxy/proxy_async.c:43:9: warning: arithmetic on a pointer to void is a GNU extension [-Wgnu-pointer-arith]
   43 |                         data += iov->iov_len;
      |                         ~~~~ ^
1 warning generated.
```

this change should address this warning by casting a `void*` pointer to
`char*` pointer before performing arithmetic operation on it.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>