Including `Windows.h` after `glad.h` raises "macro redefinition" warning
When compiling the following code snippet on Windows:

```cpp
#include <glad/glad.h>
#include <Windows.h>

int main() {}
```

I get the following warning:

```
[...]\minwindef.h(130,1): warning C4005: 'APIENTRY': macro redefinition (compiling source file [...])
[...]\include\glad/glad.h(32): message : see previous definition of 'APIENTRY' (compiling source file [...])
```

This is with glad 0.1.33, using the webservice: https://glad.dav1d.de/#profile=core&language=c&specification=gl&api=gl%3D3.0

I have enabled warnings as errors in my project, which means I need to undefine `APIENTRY` after including `glad.h` in order to have a successful build. Undefining the macro at the end of `glad.h` would prevent that problem.