Mark unsafe_pin_internals as incomplete.

This thus still makes it technically possible to enable the feature, and thus
to trigger UB without `unsafe`, but this is fine since incomplete features are
known to be potentially unsound (labelled "may not be safe").

This follows from the discussion at https://github.com/rust-lang/rust/pull/93176#discussion_r799413561

compiler/rustc_span/src/symbol.rs

@@ -1455,6 +1455,7 @@ symbols! {
         unsafe_block_in_unsafe_fn,
         unsafe_cell,
         unsafe_no_drop_flag,
+        unsafe_pin_internals,
         unsize,
         unsized_fn_params,
         unsized_locals,