Allow static implementation

From stb_howto:

Have a #define which makes function declarations and function definitions static.

Being able to add storage-class specifiers is not only useful in the context mentioned in the linked docs, but also for creating a static library that uses qoi.h but doesn't expose its symbols (see raysan5/raylib#4207), and may allow a compiler to generate better code.

For example, having implemented it in a fork and using it (and STBIDEF) in qoiconv and qoibench had their sizes drop drastically (from 216k and 234k to 75k and 109k respectively).