kernel: make _dump_ready_q() static and visible only with CONFIG_KERNEL_DEBUG

Fixes sparse warning:
<snip>/zephyr/kernel/sched.c:368:6: warning: symbol '_dump_ready_q' was not declared. Should it be static?

Change-Id: I156e89f1d74178bbd99cc25e532da544c7ebee60
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
1 file changed