attempt to fix linux build

on some older boxes, after my commit
dac4ca5 "new loplugin: externalandnotdefined"

Change-Id: Ibcab5f39714fb531066709e91a7322270995a6a5
convert sw/ObservableThread to salhelper::SimpleReferenceObject

instead of re-implementing it's own reference counting

Change-Id: I2600b970544f1dfd84b987d1891c79e3c377bd1b