lib/ctf-ir/utils.c: lazy-initialize the hash table of reserved keywords

lib/ctf-ir/utils.c: lazy-initialize the hash table of reserved keywords

This fixes bugs in static build context on platforms where the calling
order of library constructors is undefined: this constructor could be
called before GLib's constructor, in which case you cannot call
g_hash_table_new() because GLib's slice allocator is not initialized.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>