[llvm-readobj] - Always print "Predecessors" for version definition sections.

This is a follow-up discussed in D70495 thread.

The current logic is unusual for llvm-readobj. It doesn't print predecessors
list when it is empty. This is not good for machine parsers.
D70495 had to add this condition during refactoring to reduce amount of changes,
in tests, because the original code also had a similar logic.

Now seems it is time to get rid of it. This patch does it.

Differential revision: https://reviews.llvm.org/D70717