Add required cclib to link with parmap.cma

When linking statically against parmap, one need to link with
parmap_stubs library (since it's not recorded in the cma itself).
Findlib can automatically add the required -cclib option.

Note that there may be a much better way to fix this - like,
have this -lparmap_stubs recorded in the "Extra C object files"
header of parmap.cma (like it is in parmap.cmxa).