samples: philosophers: fix wrong assert type

zephyrproject/zephyr/samples/philosophers/src/main.c: In function
'philosopher':

zephyrproject/zephyr/include/zephyr/sys/__assert.h:44:52: warning:
format '%u' expects argument of type 'unsigned int', but argument 2 has
type 'stack_data_t' {aka 'long unsigned int'} [-Wformat=]

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>