common: install ncdu as an essential tool

roles/common/tasks/main.yml

 ---
 
 - name: install essential tools
-  pacman: name=vim,nano,tmux,htop,bash-completion,rsync,vnstat state=present
+  pacman: name=vim,nano,tmux,htop,ncdu,bash-completion,rsync,vnstat state=present
 
 - name: start and enable vnstatd
   service: name=vnstat enabled=yes state=started