Update library current version due to adding destroy API

Update library current version due to adding destroy API

Both current and age need to be updated when adding an API.

Luckily, liburcu 0.9.0 has a 4:0:0 (current=4) soname version, which
skipped 3:0:0. Therefore, use this in-between current version (3) to
indicate the introduction of this new queue/stack destroy API.

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