lttng: Fix a bug with lttng version when compiled from source

The output of the lttng version command is something like this:

lttng version heads/stable-2.5-0-g3910e60 - Fumisterie

The current code didn't recognize this as version.

Change-Id: I77362cf129752c2fd3301d9e60b671fa024ff1b5
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/35495
Reviewed-on: https://git.eclipse.org/r/38693
Tested-by: Hudson CI