archivers/urbackup-client: Fix for clang/libc 19

This is a fix for compiler changes in the latest LLVM versions,
including "auto_ptr" and the "std::char_traits" template removal.
This works for FreeBSD 15-CURRENT as well as previous supported
versions of FreeBSD.

PR: 286213
MFH: 2025Q2
(cherry picked from commit 0156537c1e7bf21ac3f9ed17c5d5154725729b22)