Cargo automatically imports proc_macro in Rust 2018, but Soong only
does this for rust_proc_macro targets.

--- a/src/lib.rs	2023-11-06 16:32:46.480193187 +0100
+++ b/src/lib.rs	2023-11-06 16:29:38.743132168 +0100
@@ -34,6 +34,8 @@
 use crate::mock_item_struct::MockItemStruct;
 use crate::mockable_item::MockableItem;
 
+extern crate proc_macro;
+
 // Define deterministic aliases for these common types.
 type HashMap<K, V> = std::collections::HashMap<K, V, BuildHasherDefault<std::collections::hash_map::DefaultHasher>>;
 type HashSet<K> = std::collections::HashSet<K, BuildHasherDefault<std::collections::hash_map::DefaultHasher>>;