libdumputils: avoid set<const T>

A container of const T uses std::allocator<const T>, which was an
undocumented libc++ extension that has been removed.

See https://github.com/llvm/llvm-project/pull/96319.

Bug: 349681543
Test: m libdumputils
Change-Id: Ie673d1f16ebe25050e4f04a2fce797cb6a98f2d9