fix: make sure the const-extern-fn feature is enabled

This was dropped in fa554bc on `main` and 674cc1f on `libc-0.2`
("Drop the libc_const_extern_fn conditional"). Unfortunately, this meant
that functions were incorrectly never getting marked `const`.

Instead of the fix here, I attempted to just use `cfg(not(libc_ctest))`
instead of a Cargo feature to enable `const` extern functions; however,
this seemed extremely problematic with `ctest` for some reason [1].
Instead, leave the feature as-is and just make it enabled by default.

Fixes: rust-lang#4115

[1]: rust-lang#4134

Cargo.toml

@@ -135,7 +135,7 @@ cargo-args = ["-Zbuild-std=core"]
 rustc-std-workspace-core = { version = "1.0.0", optional = true }
 
 [features]
-default = ["std"]
+default = ["const-extern-fn", "std"]
 std = []
 rustc-dep-of-std = ["rustc-std-workspace-core"]
 extra_traits = []