console: missing kernel.h include in header

[lemrey]

console.h requires k_fifo but kernel.h is not included.
CC @pfalcon.

[pfalcon]

@lemrey : How did you hit it? (Generally, we should back such things with regression tests. Maybe we'll be too lazy to write one, but we should not be lazy to discuss how to write one ;-).

[lemrey]

@pfalcon I hit a compiler warning; perhaps it is possible to reproduce by including console.h before kernel.h (or by not including kernel.h at all?): warning: ‘struct k_fifo’ declared inside parameter list will not be visible outside of this definition or declaration

[pfalcon]

Thanks. Fairly speaking, I had an idea that all Zephyr apps should start with #include <zephyr.h>. Dunno where I got the from though, probably because all in-tree samples do that. Anyway, it's an easy fix, posted as #9541