Silence private_bounds lint in KnownLayout derive

For reasons not-yet-known, the `private_bounds` lint flags the
output of `derive(KnownLayout)` on `repr(C)` structs generated
by macros. This is likely a rustc bug, but to preserve a good
experience for our users, we nonetheless, `allow(private_bounds)`.

Fixes #2177

zerocopy-derive/src/lib.rs

@@ -320,6 +320,9 @@ fn derive_known_layout_inner(ast: &DeriveInput, _top_level: Trait) -> Result<Tok
                 // `#ty`, not `__ZerocopyKnownLayoutMaybeUninit` (see #2116).
                 #repr
                 #[doc(hidden)]
+                // Required on some rustc versions due to a lint that is only
+                // triggered on macro-generated code. See #2177 for details.
+                #[allow(private_bounds)]
                 #vis struct __ZerocopyKnownLayoutMaybeUninit<#params> (
                     #(::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<
                         <#ident #ty_generics as