Move fd-tracker to liblttng-ust-common

The fd-tracker provides ABI symbols to liblttng-ust-ctl which shouldn't
link directy on liblttng-ust, move it to liblttng-ust-common to
accomodate this. This is in line with the goal to move the shared state
across our different libraries to ust-common.

Change-Id: I69de6a9e6eaec9599f22e47cb0292ef793f6115d
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>