Fix bevy_window failing with serialize feature (#14298)

# Objective

- [`flag-frenzy`](https://github.com/TheBevyFlock/flag-frenzy) found an
issue where `bevy_window` would fail to build when its `serialize`
feature is enabled.
- See
[here](https://github.com/TheBevyFlock/flag-frenzy/actions/runs/9924187577/job/27415224405)
for the specific log.

## Solution

- Turns out it was failing because the `bevy_ecs/serialize` feature was
not enabled. This error can be fixed by adding the flag as a dependency.

## Testing

```bash
cargo check -p bevy_window -F serialize
# Or if you're very cool...
flag-frenzy --manifest-path path/to/bevy/Cargo.toml --config config -p bevy_window
```

crates/bevy_window/Cargo.toml

@@ -10,7 +10,7 @@ keywords = ["bevy"]
 
 [features]
 default = []
-serialize = ["serde", "smol_str/serde"]
+serialize = ["serde", "smol_str/serde", "bevy_ecs/serialize"]
 
 [dependencies]
 # bevy