freetype: darwin: avoid warning about VLA folded into array

A const size_t is not quite the same as an integer constant and recently compilers started to warn about this. We can easily avoid this by using a define, however here we can just use the size of the array itself and do not need to hardcode any size at all.

Avoids the following compiler warning with recent Clang: warning: variable length array folded to constant array as an extension