Fix: liblttng-ctl: missing index allocator symbols

Fix: liblttng-ctl: missing index allocator symbols

abidiff reports that 5 functions were erroneously removed by the 2.13.4
release. Those functions were initially erroneously exposed, but the
symbols must be maintained to preserve the ABI of liblttng-ctl.

abidiff report:
  5 Removed functions:

    [D] 'function lttng_index_allocator_status lttng_index_allocator_alloc(lttng_index_allocator*, uint64_t*)'    {lttng_index_allocator_alloc}
    [D] 'function lttng_index_allocator* lttng_index_allocator_create(uint64_t)'    {lttng_index_allocator_create}
    [D] 'function void lttng_index_allocator_destroy(lttng_index_allocator*)'    {lttng_index_allocator_destroy}
    [D] 'function uint64_t lttng_index_allocator_get_index_count(lttng_index_allocator*)'    {lttng_index_allocator_get_index_count}
    [D] 'function lttng_index_allocator_status lttng_index_allocator_release(lttng_index_allocator*, uint64_t)'    {lttng_index_allocator_release}

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I7d69a0526ccb9ba69ddfbad6955c81146bb83698