API does not provide a way to safely free qoa_decode return value

The API does not provide a way to free the memory returned by `qoa_decode()`. I would use `QOA_FREE()` but this macro is only available when defining `QOA_IMPLEMENTATION`, which does not work when using C++ (see #34). Of course there are plenty of workarounds, but they all introduce some inconsistencies, so maybe a simple `void qoa_free(void *p) { QOA_FREE(p); }` in the API could help.