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>