Ambiguous callback symbols when using non-gl debug loaders.
Say I want to load GL and WGL extensions into my application. I do this with something like this:

```cxx
auto hRC = ::wglCreateContext(hDC);
	
::wglMakeCurrent(hDC, hRC);
::gladLoadGL();
::gladLoadWGL(hDC);
```

From my understanding, this requires both loaders, i.e. `glad.c` and `glad_wgl.c` to be generated. 

However, in debug-mode, both source files contain the same definitions for `_pre_call_callback_default`/`_post_call_callback_default` and `glad_set_pre_callback`/`glad_set_post_callback`, causing errors when linking the code. 

Am I missing something or using the code in a wrong way or is this a bug?

I noticed, that the [this line](https://github.com/Dav1dde/glad/blob/7ece538856bf124d798ab323c8e1e64ebb83cb50/glad/lang/c/debug.py#L71) already writes different implementations, so an easy fix (imo) would be to prefix the function names.