Bug#1024889: pplacer: build-depedends on dropped package.

[Peter Michael Green]

Package: pplacer
version: 1.1~alpha19-6
Severity: serious
Justification: rc policy: "packages must be buildable within the same 
release"

pplacer build-depends on libmcl-ocaml-dev which is no longer built by the
mcl source package. It is still present in unstable as a cruft package, but
is completely gone from testing.