docproc: Move printk file location

> docproc: kernel/printk.c: No such file or directory

So rename the file.

Signed-off-by: Joe Perches <joe@perches.com>
Noticed-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>