Add hook file to generator dependency list

[Naros]

This should guarantee that when the hook file contents change, the files are regenerated on CMake and don't become stale across rebuilds.

FYI @Togira123

In addition to this, I do have one question. In our implementation what we opted to do was to have the hooks file act as a delegator/reader. In other words our project looks like this:

cmake/bindings/resource_loader.cpp
cmake/bindings/resource_loader.hpp
cmake/bindings/bindings_hook.py
src/<our plugin>

resource_loader.hpp

#include <godot_cpp/core/version.hpp>

//~ Begin CustomBindings Interface
#if GODOT_VERSION_MAJOR == 4 && GODOT_VERSION_MINOR < 8
String get_resource_type(const String &p_path);
#endif
//~ End CustomBindings Interface

resource_loader.cpp

#include <godot_cpp/classes/resource.hpp>
#include <godot_cpp/core/version.hpp>

//~ Begin CustomBindings Implementation
#if GODOT_VERSION_MAJOR == 4 && GODOT_VERSION_MINOR < 8
String ResourceLoader::get_resource_type(const String &p_path) {
	const Ref<Resource> resource = load(p_path, "", CACHE_MODE_IGNORE);
	if (resource.is_valid()) {
		return resource->get_class();
	}

	return String();
}
#endif
//~ End CustomBindings Implementation

The script knows how to parse these fragments and place the contents where it belongs.

Our CMAKE module watches for any changes to the contents in cmake/bindings/** and if a file changes, this enforces a similar situation to what you see here by adding the hook file as a dependency.

I do wonder whether it would make sense for there to be something like a GODOTCPP_BINDING_HOOK_DIR and here is where a user would place their python script and any other content, which would be treated as a DEPENDS input here, and any content that changes in this directory enforces a regeneration pass. The other option is just a generic GODOTCPP_BINDING_DEPENDS where the extension supplies a list of files, which could include the python script and any other dependencies.

For now, we have to generate a SHA256 fingerprint on each file in the directory, and if there are changes, we remove the godot-cpp hpp/cpp files so they get regenerated, which seems like it would be naturally expected to work that way out of the box.