book: Add instruction to add once_cell

Author: Hofer-Julian

book/src/g_object_signals.md

@@ -34,6 +34,12 @@ First we override the `signals` method in `ObjectImpl`.
 In order to do that, we need to lazily initialize a static item `SIGNALS`.
 Until [`std::sync::LazyLock`](https://doc.rust-lang.org/std/sync/struct.LazyLock.html) is stabilized, we use [`once_cell::sync::Lazy`](https://docs.rs/once_cell/latest/once_cell/sync/struct.Lazy.html) instead.
 
+This also means, we have to add the `once_cell` crate as dependency by executing:
+
+```
+cargo add once_cell
+```
+
 Filename: <a class=file-link href="https://github.com/gtk-rs/gtk4-rs/blob/master/book/listings/g_object_signals/2/custom_button/imp.rs">listings/g_object_signals/2/custom_button/imp.rs</a>
 
 ```rust ,no_run,noplayground