lttng-modules: bump to version 2.8.3

LTTng-modules, starting from v2.8, has a mechanism to be built directly
into a Linux kernel image, that is, not as loadable kernel modules. For
this the `CONFIG_LTTNG_CLOCK_PLUGIN_TEST` and `CONFIG_LTTNG` Make
variables were introduced. They need to be explicitly set to `m` here to
build the project as loadable kernel modules.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>