[LLDB][Python] fix another fflush issue on NetBSD

Summary:
Here's another instance where we were calling fflush on an input
stream, which is illegal on NetBSD.

Reviewers: labath, mgorny

Reviewed By: mgorny

Subscribers: krytarowski, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D69488