[llvm-readobj] - Fix a warning.

In a post review comments for D88097 it was mentioned that code
triggers bunch of warnings of the form:

llvm/tools/llvm-readobj/ELFDumper.cpp:5299:28: warning: loop variable 'Note' is always a copy because
the range of type 'iterator_range<llvm::object::ELFFile<llvm::object::ELFType<llvm::support::big, true> >::Elf_Note_Iterator>'
(aka 'iterator_range<Elf_Note_Iterator_Impl<ELFType<(llvm::support::endianness)0U, true> > >') does not return a reference [-Wrange-loop-analysis]
      for (const Elf_Note &Note : this->Obj.notes(P, Err))

It happens because Elf_Note is always copied here:

Elf_Note_Impl<ELFT> operator*() const {
  assert(Nhdr && "dereferenced ELF note end iterator");
  return Elf_Note_Impl<ELFT>(*Nhdr);
}

This patch fixes the issue by removing a reference.