opam: add dependency on bubblewrap.

[CMB]

[chris@cmbvm ~]$ opam init
[NOTE] Will configure from built-in defaults.
Checking for available remotes: rsync and local, git.

• you won't be able to use mercurial repositories unless you install
  the hg command on your system.
• you won't be able to use darcs repositories unless you install
  the darcs command on your system.

[ERROR] Missing dependencies -- the following commands are required
for opam to operate:

• bwrap: Sandboxing tool bwrap was not found. You should install
  'bubblewrap'. See
  https://opam.ocaml.org/doc/FAQ.html#Why-does-opam-require-bwrap.

General

• This is a new package and it conforms to the quality requirements

Have the results of the proposed changes been tested?

• I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
• [ x] I generally don't use the affected packages but briefly tested this PR