Fix: bindings import segfaults on missing hash_key_u64

A mishandled error in SWIG-generated code causes the Python3
interpreter to segfault when a missing symbol is reported during
the linking (at runtime) against liblttng-ctl.

libcommon makes use of the internal libhashtable.la since the
addition of the lttng_trace_chunk interface. This introduces
a transitive dependency to libhashtable.la in liblttng-ctl.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>