Fix: Only save kernel enablers in session configuration

The session configuration serialization currently saves the
kernel enablers along with all enabled syscalls. In the case
where a syscall would be enabled with a given filter, this would
result in two events being enabled:

1) the syscall with its filter expression (the enabler)
2) the syscall on its own (an enabled syscall)

The observable effect of this is that the syscall ends up being
traced regardless of the filter's evaluation.

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