Add extern to global struct variables in GDExtension C example

[dsnopek]

Fixes #11387

[charjr]

As well as adding extern in api.h they also have to then be defined in api.c like in vnen's example https://github.com/vnen/gdextension-c-example/blob/b560e60ab5f7f6bfd4b85e71cd68ad736562ebdf/src/api.c#L5-L9

Lastly, this change #11385.

Vnen's examples use include guards, whereas the docs are using #pragma once. defs.h is documented with #pragma once at the top and then #endif //DEFS_H at the bottom. So one or the other should be changed.

[dsnopek]

As well as adding extern in api.h they also have to then be defined in api.c like in vnen's example https://github.com/vnen/gdextension-c-example/blob/b560e60ab5f7f6bfd4b85e71cd68ad736562ebdf/src/api.c#L5-L9

Yep, that's already in this PR