samples: philosophers: fix pointer type mismatch for stack fork

When using the stack fork type in the philosophers sample,
there were a few pointer type mismatch since the k_stack_*()
functions are using stack_data_t but the parameters are uint32_t.
So cast them to void pointer to suppress compiler warnings.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
1 file changed