[llvm-readobj] - Add 'override' to fix build bots.

This should fix bots after landing D93900.

An example of error is:

/home/worker/2.0.1/lldb-x86_64-debian/llvm-project/llvm/tools/llvm-readobj/ELFDumper.cpp:883:8: warning: 'printSectionMapping' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  void printSectionMapping() {}