Add a catch for using different header files depending on linux kernel version

[lhovo]

Adding a catch to use the correct signal header file based on the linux kernel version.
Newer linux kernels now use linux/sched/signal.h where it use to be linux/signal.h

This patch allows the code to be compiled on both linux versions and find the correct location without compile error.

[sjlongland]

Silly question, what exactly does this do, and what exactly is a kernal?

Ultimately I'd like to clean this driver up and have it merged into the Linux kernel (where it ought to have been from day one) but I can guarantee they'll reject any such patch set with a non-descriptive commit such as this.

[lhovo]

Apologies, it was a late night trying to get my hardware working.
I have updated the description and the comment.