collectd: make pinba support a selectable feature

Pinba support requires protobuf-c that is currently only available in
meta-virtualization layer. Make this a selectable feature.

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>