Including glad.h in multiple files causes build error
In order to use GL functions in more than one cpp file, I have included `glad.h` in two separate cpp files. Without glad.h, the `gl<whatever>` functions are undefined and cause a build error, but if I include glad.h in both files, the following code from `glad.h` triggers an error.
```
#ifdef __gl_h_
#error OpenGL header already included, remove this include, glad already provides it
#endif
#define __gl_h_
```

My application builds and works fine if I comment out the `#error` line