Introduce EXTRA_VERSION_PATCHES

This allow third-party (packagers) to provide more information about
what custom patches were applied to the tree.

To do so, one can create emtpy files in "version/extra_patches/",
the filenames will be used to generate the EXTRA_VERSION_PATCHES
define.

Add this information to the debug log of lttng-relayd and lttng-sessiond.

Also append it at the end of the "version" command of the lttng binary.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>