Fix compile error caused by incorrect feature flag in bevy_state (#…

…14987)

# Objective

The `reflect` module in `bevy_state` is gated behind the `bevy_reflect`
feature, but the type exports from that module in the crate prelude are
erroneously gated behind the `bevy_app` feature, causing a compile error
when the `bevy_reflect` feature is disabled, but the `bevy_app` feature
is enabled.

## Solution

Change the feature gate to `bevy_reflect`.

## Testing

- Discovered by depending on `bevy_state` with `default-features =
false, features = ["bevy_app"]`
- Tested by running `cargo check -p bevy_state --no-default-features
--features bevy_app`

crates/bevy_state/src/lib.rs

@@ -54,7 +54,7 @@ pub mod prelude {
  pub use crate::app::AppExtStates;
  #[doc(hidden)]
  pub use crate::condition::*;
- #[cfg(feature = "bevy_app")]
+ #[cfg(feature = "bevy_reflect")]
  #[doc(hidden)]
  pub use crate::reflect::{ReflectFreelyMutableState, ReflectState};
  #[doc(hidden)]