Source lib/dstat in unstack.sh

`stop_dstat` function was extracted into $TOP_DIR/lib/dstat in commit
e0b08d04ab41f9aca96997c87bd9fa13feba2536 but the new lib was not sourced in
unstack.sh.

Change-Id: I6878552d9f384fdd7882e52000b37ac6a60844c3
Closes-Bug: 1366713
Merge "Source lib/dstat in unstack.sh"