[flang] Fix bug in writing PRIVATE subprograms to .mod file

Subprograms can't have the PRIVATE prefix on them when they are defined.
So if they are private, add a private-stmt for them.

Fixes flang-compiler/f18#519

Original-commit: flang-compiler/f18@d3670aa176cbc79a7d26fc927e9d1e1349900756
Reviewed-on: https://github.com/flang-compiler/f18/pull/523