Fix: incorrect structure layout with mixed LGPL/non-LGPL objects

Linking both _LGPL_SOURCE and non-_LGPL_SOURCE objects into the same
module may result in corruption. If the tracepoint_dlopen object used is
the one declared by a LGPL compile unit, a non-LGPL compile unit may try
to initialize fields beyond the end of the structure.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>