Fix printk() header inclusion for older kernels

This was tickled when `xeno_math` became the default math kmodule for
RTAI in #235, and Janos Bujtar tried building against the older kernel
packages distributed by the LinuxCNC project ([reported here][1]).

`printk()` was moved from `linux/kernel.h` to `linux/printk.h` in
[late 2010][2].  However, `linux/kernel.h` includes `linux/printk.h`.

Alternatively, `printk()` could be defined by including
`linux/module.h`.  I don't know which is better.

[1]: https://groups.google.com/forum/#!topic/machinekit/PXjZfJqOfPg
[2]: https://lkml.org/lkml/2010/11/9/606