fix typo in opl.h

[BLumia]

I am reusing the opl.h in my side project for fun, and found this missing extern C issue. This patch adds the missing extern C to make C++ project reuse the opl.h as a header-only library easier. （edit: removed changes related to this according to #45 (comment))

And also corrected a typo.

[mattiasgustavsson]

Hmm, I usually don't add "extern C" to my libraries, as they are designed to be compilable with both C and C++, the idea being that if you use them from C, you include implementation from a .c file, and if you use them from C++, you include implementation from a .cpp file. Did you try including implementation in a .cpp file? Did you run into any compilation errors? If so, I would rather fix those than add extern C.

[BLumia]

the idea being that if you use them from C, you include implementation from a .c file, and if you use them from C++, you include implementation from a .cpp file.

Oh, that's okay. I indeed include implementation from a .c file but use them in .cpp file, which lead this issue. I added the extern C because libraries like tml.h, tsf.h, dr_wav.h which are used by this project also did the same thing.

Should I update the patch to only include the typo fix, or simply close this PR?

Thanks!

[mattiasgustavsson]

Up to you, I can fix the typo, or if you do it I'll merge the PR

[BLumia]

Thanks! PR amended :)

[mattiasgustavsson]

Thanks!