Fix GDExtension build error: incomplete type 'godot::Variant'

[Nordup]

I was getting this error while compiling GDExtension

In file included from generators/graph/voxel_generator_graph.h:7,
                 from generators/graph/voxel_generator_graph.cpp:1:
generators/graph/../../util/godot/core/dictionary.h: In function 'bool zylann::godot::try_get(const godot::Dictionary&, const godot::Variant&, T&)':
generators/graph/../../util/godot/core/dictionary.h:26:17: error: 'v' has incomplete type
   26 |         Variant v = d.get(key, Variant());
      |                 ^
In file included from generators/graph/../../util/godot/core/dictionary.h:7:
godot-cpp/gen/include/godot_cpp/variant/dictionary.hpp:43:7: note: forward declaration of 'class godot::Variant'
   43 | class Variant;
      |       ^~~~~~~
generators/graph/../../util/godot/core/dictionary.h:29:38: error: incomplete type 'godot::Variant' used in nested name specifier
   29 |         if (v.get_type() == Variant::NIL) {
      |                                      ^~~

Suggesting that Variant type is only forward declared in dictionary.hpp, but does not include its full definition.

[Zylann]

In this case I would probably move the try_get function implementation to a cpp file then. If Variant is forward-declared, I think it should remain so to avoid including Variant in all other headers that would not need its definition.