Fixed a magic constant in FTW stub.
Fixed a magic constant in FTW stub.

The stub for the C summarize tree solution using the `ftw()` system call had a magic constant (16) just sitting there with no explanation. I used `#define` to give it a name (`MAX_FTW_DEPTH`).
UMM-CSci-Systems/Traversing-Directories-starter,UMM-CSci-Systems/Traversing-Directories-starter