Fix: liblttng-ctl: hide symbols introduced by packing fixes

Fix: liblttng-ctl: hide symbols introduced by packing fixes

abidiff reports the following when comparing the ABIs of
liblttng-ctl of v2.13.0 to the stable-2.13 branch.

  12 Added functions:

    [A] 'function ssize_t lttng_event_context_create_from_payload(lttng_payload_view*, lttng_event_context**)'    {lttng_event_context_create_from_payload}
    [A] 'function void lttng_event_context_destroy(lttng_event_context*)'    {lttng_event_context_destroy}
    [A] 'function int lttng_event_context_serialize(lttng_event_context*, lttng_payload*)'    {lttng_event_context_serialize}
    [A] 'function ssize_t lttng_event_create_from_payload(lttng_payload_view*, lttng_event**, lttng_event_exclusion**, char**, lttng_bytecode**)'    {lttng_event_create_from_payload}
    [A] 'function ssize_t lttng_event_field_create_from_payload(lttng_payload_view*, lttng_event_field**)'    {lttng_event_field_create_from_payload}
    [A] 'function int lttng_event_field_serialize(const lttng_event_field*, lttng_payload*)'    {lttng_event_field_serialize}
    [A] 'function lttng_error_code lttng_event_fields_create_and_flatten_from_payload(lttng_payload_view*, unsigned int, lttng_event_field**)'    {lttng_event_fields_create_and_flatten_from_payload}
    [A] 'function int lttng_event_serialize(const lttng_event*, unsigned int, char**, char*, size_t, lttng_bytecode*, lttng_payload*)'    {lttng_event_serialize}
    [A] 'function lttng_error_code lttng_events_create_and_flatten_from_payload(lttng_payload_view*, unsigned int, lttng_event**)'    {lttng_events_create_and_flatten_from_payload}
    [A] 'function lttng_rate_policy_status lttng_rate_policy_every_n_get_interval(const lttng_rate_policy*, uint64_t*)'    {lttng_rate_policy_every_n_get_interval}
    [A] 'function lttng_rate_policy_status lttng_rate_policy_once_after_n_get_threshold(const lttng_rate_policy*, uint64_t*)'    {lttng_rate_policy_once_after_n_get_threshold}
    [A] 'function lttng_session_descriptor_status lttng_session_descriptor_get_session_name(const lttng_session_descriptor*, const char**)'    {lttng_session_descriptor_get_session_name}

The packing fix patch series exposes a number of internal symbols
through liblttng-ctl. Mark those functions as hidden.

Note that the three last functions of this report were correctly
exposed by fixes following the 2.13.0 release.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Idf98f9d22388f945913c801a3da966167e7c3f3f