os_types.h: Properly define types on macOS

[fgiancane8]

Hello!

I was trying to build ogg vorbis flac opus and sndfile libraries for macOS. Unfortunately, with more recent builds of the macOS operating systems, it seemed that some includes were missing.

Digging deeper into the problem, I found that actually the culprit was os_types.h, not including <stdint.h>. Adding the include fixed the issue.

ogg was built using cmake on a Big Sur macOS and latest XCode tools.