lttng: Enable "potential resource leak" warnings

lttng: Enable "potential resource leak" warnings

These warnings were disabled, because they often were a bit annoying
to fix with pre-Java 7 syntax. Now that we can use try-with-resources,
those should be as straightforward as the "definite resource leak"
warnings. So we should enable them, and eventually work on fixing them!

Change-Id: Id1b2e63ff6908ca8e34fc07079853922554a0e22
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/26305
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Hudson CI