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
diff --git a/unstack.sh b/unstack.sh
index 0457ef2..adb6dc1 100755
--- a/unstack.sh
+++ b/unstack.sh
@@ -55,6 +55,7 @@
 source $TOP_DIR/lib/neutron
 source $TOP_DIR/lib/baremetal
 source $TOP_DIR/lib/ldap
+source $TOP_DIR/lib/dstat
 
 # Extras Source
 # --------------