Disable the unexpected_cfgs warning for now.

Telling rustc about `doc_cfg` would require a build.rs script, which
I'd rather not add if we don't otherwise need one. So for now, just
disable this warning.

src/lib.rs

@@ -9,6 +9,7 @@
 #![feature(strict_provenance)]
 #![feature(exposed_provenance)]
 #![deny(fuzzy_provenance_casts, lossy_provenance_casts)]
+#![allow(unexpected_cfgs)]
 #![no_std]
 
 #[cfg(all(feature = "alloc", not(feature = "rustc-dep-of-std")))]