Fix: liblttng-ctl: non-packed structure used for tracker serialization

Fix: liblttng-ctl: non-packed structure used for tracker serialization

Using unpacked structures in liblttng-ctl's protocol can cause issues
when mixing sessiond and client of different bitness. In this specific
case I doubt it causes a problem, but it could rightfully do on some
architectures.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie17096a55a4b7508c604e184cae877b83df6e451