Add file "export.hpp"

[Fabio Krapohl]

When including this, the file gl/export.hpp is not found.
You probably forgot to git add it, because it's not in the directory include/gl/

[Ali Can Demiralp]

Hello, cmake automatically creates this file once you build the library. I have also added it to the repository now for convenience.

[Fabio Krapohl]

No, didn't help. Just including gives following error now (using clang++):

/usr/local/include/gl/opengl.hpp:16:1: error: unknown type name '__declspec'
GL_EXPORT bool initialize ();
^
/usr/local/include/gl/export.hpp:15:25: note: expanded from macro 'GL_EXPORT'
#      define GL_EXPORT __declspec(dllimport)
                        ^