Added -printer o to the arguments of camlp4o when --pp is used in
piqic-ocaml

Camlp4o uses the Camlp4AutoPrinter by default (no -printer argument
provided), which will either produce ocaml code or a binary AST
depending on whether the output is a terminal or not (regardless of any
-o option provided). If piqic-ocaml is run from a process that is not
started on a terminal, the results of piqic-ocaml --pp was a binary AST
file.