samples: cmsis_rtos_v1: philosophers: increas CMSIS_THREAD_MAX_STACK_SIZE

This sample is failing regularly in CI and locally for
qemu_cortex_a53_smp, seems like the stack is not enough:

thread_analyzer: 0x4001f740: STACK: unused 1040 usage 1008 / 2048 (49 %)
thread_analyzer: 0x4001f3a0: STACK: unused 944 usage 1104 / 2048 (53 %)
thread_analyzer: 0x4001f000: STACK: unused 1040 usage 1008 / 2048 (49 %)
...

Increasing to 2048 seems to make it run reliably.

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