kernel APIs have changed to using K_THREAD_STACK_DEFINE to declare stack
memory as well as K_THREAD_STACK_SIZEOF to calculate it's size.
Adjust irc_bot sample to reflect those changes.
This fixes stack related page faults when running irc_bot with
CONFIG_X86_STACK_PROTECTION enabled (which it is by default for qemu).
Signed-off-by: Michael Scott <michael.scott@linaro.org>