Work well with older lock_api versions

The crate `lock_api` is a transitive dependency, pulled in trough the
use of `parking_lot`. `parking_lot` depends on `lock_api` version
`^0.4.6`, but only since `lock_api` 0.4.7 the `RwLock::new()` method is
available on rust stable, coinciding with the release of rust
[1.61][const_fn_trait_bound].

A problem occurs if you try to use `once_map` with non-up-to-date
dependencies, e.g. by running

```sh
RUSTC_BOOTSTRAP=1 cargo generate-lockfile -Zminimal-versions
```

This PR adds an explicit dependency to `lock_api` 0.4.7. The dependency
is still optional and is only pulled in through feature `std`.

Cargo.toml

@@ -17,7 +17,7 @@ keywords = ["map", "once", "lazy"]
 [features]
 default = ["std", "ahash"]
 
-std = ["dep:parking_lot", "stable_deref_trait/std", "ahash?/std", "ahash?/runtime-rng"]
+std = ["dep:parking_lot", "dep:lock_api", "stable_deref_trait/std", "ahash?/std", "ahash?/runtime-rng"]
 ahash = ["dep:ahash", "hashbrown/ahash"]
 
 rayon = ["dep:rayon", "hashbrown/rayon", "std"]
@@ -31,6 +31,7 @@ equivalent = { version = "1.0", optional = true }
 
 parking_lot = { version = "0.12", optional = true }
 rayon = { version = "1.6", optional = true }
+lock_api = { version = "0.4.7", optional = true }
 
 
 [package.metadata.docs.rs]