Fix: liblttng-ust-ctl: missing ring buffer and counter clients init/exit symbols

Fix: liblttng-ust-ctl: missing ring buffer and counter clients init/exit symbols

Move init/exit functions to ust-core.c which is linked against by both
liblttng-ust and liblttng-ust-ctl to make them available to the control
library.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I70bd2841c941df8e4d25629a0747aaebe57e0569