Missing end } for C++ header files

[Svengali]

In the header file SDL_gpu_shadercross.h, theres a missing

`/ Set up for C function definitions, even when using C++ /

ifdef __cplusplus

}

endif

`

I added it locally, and can submit a PR, but just wanted to point it out

[flibitijibibo]

I can merge a PR for this if you have it working locally!

[flibitijibibo]

Fixed by #42