common: install compsize for use with btrfs rootfs

I have needed to use compsize on multiple occasions, and thus had to
temporarily install it. As it is already installed on 9 machines and
is generally useful, make it part of the standard tool set for btrfs.

roles/common/tasks/main.yml

 - name: Install essential tools
   pacman: name=vim,nano,tmux,htop,ncdu,bash-completion,rsync,vnstat state=present
 
+- name: Install compsize for use with btrfs
+  pacman: name=compsize state=present
+  when: filesystem == "btrfs"
+
 - name: Start and enable vnstatd
   service: name=vnstat enabled=yes state=started