lttng: Consider compilation warnings to be errors

lttng: Consider compilation warnings to be errors

Now that we have eliminated all compiler warnings in the code,
we could turn on "-Werror" to make sure no new warnings are
introduced (Hudson CI would fail building any Gerrit patch that
has warnings).

Only the NLS warnings remain as warnings, so that they can be
suppressed in Messages or constants classes.

Change-Id: I53394af9add34f3757f726ab773d445c92d8523e
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/14206
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>