ast_common.hpp uses std::istringstream, but does not include <sstream>.

This caused an XDM build failure on my system for the first time in a long time. It used to build just fine this way, but something must have changed in a boost update that removed some upstream include of that header from boost headers. My last build used boost 1.83, but my system updated to boost 1.84 today and that caused xdm to have a missing dependent library. Trying to rebuild to fix that exposed the problem.

Inserting an include of this header into that file fixes the problem for me. Inserting it immediately after the include of <unordered_map> did the trick for me.

Tagging @Karlsefni2012, @gjtempl, and @TBird2001 directly to make sure they see this.