fix: g_hash_table_insert prior to glib 2.40 returns void

Even the post glib 2.40 version will only return false if the value is
already present in the hashtable but will not fail to add it. The old
value will be handled according to the configuration of the hashtable
which should not be an error.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: I5512c921ce220bf8a956c61f594d6a27e6ceb41e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1247
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>