This PR fixes a warning that was present in release mode, which was
preventing the nightly builds from running:

```
error: variable does not need to be mutable
   --> crates/gpui2/src/elements/div.rs:547:9
    |
547 |     let mut div = Div {
    |         ----^^^
    |         |
    |         help: remove this `mut`
    |
    = note: `-D unused-mut` implied by `-D warnings`
```

Release Notes:

- N/A