[mlir] mlir-opt: print a newline after the top-level module

[mlir] mlir-opt: print a newline after the top-level module

A printer refactoring removed automatic newline printing in the printer
of a ModuleOp. As a consequence, mlir-opt no longer printed a newline
after the closing brace of a module, which made it hard to distinguish
when used from command line. Print the newline character explicitly in
mlir-opt.