Extras: Add a checkpatch git pre-commit hook

This commit add the checkpatch-hook script that can be installed in git
hooks folder in order to detect style issue before commiting any staged
changes.

The .checkpatch.conf file contains the appropriate options conforming to
the coding style used in the lttng-tools tree.

Also included in this commit is an install script to automatically
install the hook in the git local repo.

To override the pre-commit hook check, one can use the
`git commit --no-verify` command.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>