Samples: Philosophers: Revert stack size change

In commit 31e0d864, the stack size
was unjustifiably changed from a base of *768*
(plus a configurable extra stack size) to *2048*.
This change broke our sample tests for one of our products, which has
less available memory for the threads' stacks.

The stack size is dropped to *1024* to align with the stack sizes
specified in the other samples, which were changed in commit
31e0d864.

Signed-off-by: Saku Rautio (Ext) <saku.rautio@nordicsemi.no>