Adapt to namespaced liblttng-ust-ctl symbols

The liblttng-ust ABI bump gave us the opportunity to namespace all
public symbols under the 'lttng_ust_' prefix. All of liblttng-ust-ctl
public symbols were renamed from 'ustctl_' to 'lttng_ust_ctl_'. Adapt
the tools codebase for this API change.

Depends-on: lttng-ust: Ic2b9da019e7776b24b98824ae63d16b86c5a5742

Change-Id: I64b61a7fad86a1c0b4734a4a181f6ea7a9de7e24
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>