core: only store the stack size for DEVELHELP (implementation)

`tcp_t::stack_size` is only examined by the shell command `ps` and
`DEBUG_PRINT`. For the latter one only if `DEVELHELP` was enabled.

This PR guards the member `tcp_t::stack_size` in `#ifdef DEVELHELP`.
Only if DEVELHELP was activated its value get printed by `ps`.

Closes #1287.