Fix: syscalls-extractor: kallsyms_lookup_name no longer available

Fix: syscalls-extractor: kallsyms_lookup_name no longer available

Since v5.7, kallsyms_lookup_name is no longer available.

In order to re-use the kallsyms wrapper, it is simpler to move the
syscalls extractor module to the `src` directory and gate its
compilation behind a new config option, CONFIG_LTTNG_SYSCALLS_EXTRACTOR.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I55d878dce55827d61035693aaf5865af3c4e775f