allow the crate to refer to itself as pin-init in doc tests

The `syn` approach requires use of `::pin_init::...` instead of the
`$crate::...` construct available to declarative macros. To be able to
use the `pin_init` crate from itself (which includes doc tests), we have
to declare it as such.

Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

src/lib.rs

@@ -290,6 +290,8 @@ use core::{
     ptr::{self, NonNull},
 };
 
+extern crate self as pin_init;
+
 #[doc(hidden)]
 pub mod __internal;
 #[doc(hidden)]