samples: cmsis: philosophers: rename 'fork' to avoid POSIX conflict

There is a macro, `fork()`, inside of the philosophers example that
conflicts with the POSIX `fork()` function.

Since it is customary to avoid using reserved names, adjust the sample
so that the macro in question is called `philosopher_fork()`.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>