pw_tokenizer: Add #[used] to token Rust database entries

Change-Id: Iddfb07fd8bdb6c3d85035f6b19f2c011821b98d4
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/189293
Commit-Queue: Erik Gilling <konkers@google.com>
Reviewed-by: Aaron Green <aarongreen@google.com>

pw_tokenizer/rust/pw_tokenizer_macro.rs

@@ -73,6 +73,7 @@ fn token_backend(domain: &str, string: &str) -> TokenStream2 {
             // This is currently manually verified to be correct.
             // TODO: b/287132907 - Add integration tests for token database.
             #[link_section = #section ]
+            #[used]
             static #ident: TokenEntry = TokenEntry {
                 magic: #TOKENIZER_ENTRY_MAGIC,
                 token: #hash,