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`, which
showed up with `CMSG_SPACE` [1].

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 [2].
Instead, leave the feature as-is and just make it enabled by default.

Fixes: rust-lang#4115 [1]

[2]: rust-lang#4134

(backport <rust-lang#4134>)
(cherry picked from commit 9f23a63)

Cargo.toml

@@ -137,7 +137,7 @@ 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 = ['align', 'rustc-std-workspace-core']
 extra_traits = []