misc: _char_out can be a static symbol

Fixes sparse warning:
<snip>/zephyr/zephyr/misc/printk.c:50:5: warning: symbol '_char_out' was not declared. Should it be static?

Change-Id: I5af0860e9f8f827002ae9a142b5924d3de8d51b6
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>