webos_lttng.bbclass: Add lttng-modules to build when LTTng is enabled

:Release Notes:
Enable kernel LTTng tracing by default in LTTng builds.

:Detailed Notes:
Enable kernel LTTng tracing by default in LTTng builds by adding
lttng-modules to runtime dependencies.
Although LTTng kernel tracing is a significant chunk of relatively
experimental code added to the kernel, there are two layers of
protection to keep this from having a customer impact:
1) LTTng is not enabled in production builds. This is only available
in prerelease builds.
2) Although the lttng kernel modules are installed in the system image,
they are not insmoded at boot. The only time they are installed in
the kernel is when a developer explicitly starts using LTTng userspace
tools (e.g. "lttng create")

:Testing Performed:
Informal testing but haven't done miniBAT yet.

:QA Notes:

:Issues Addressed:
[BHV-4261] State machine based analysis of PmTrace traces
[BHV-7397] CCC: webos_lttng.bbclass: Add lttng-modules to build when LTTng is enabled

Open-webOS-DCO-1.0-Signed-off-by: Martin Unsal <martin.unsal@lge.com>
Change-Id: I5a2d84817e1155b398414255cbefea6d324ec443
Reviewed-on: https://g2g.palm.com/5603
Reviewed-by: DCO Verification
Build: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>