libcephfs_proxy: remove arithmetic on void*

this change is created in the same spirit of bb1fa81.

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 <[email protected]>

Author: tchaikov

src/libcephfs_proxy/proxy_async.c

@@ -40,7 +40,7 @@ static int32_t libcephfsd_cbk_nonblocking_rw(proxy_async_t *async,
 			}
 
 			memcpy(iov->iov_base, data, iov->iov_len);
-			data += iov->iov_len;
+			data = (char*)data + iov->iov_len;
 			size -= iov->iov_len;
 			iov++;
 			count--;