Fix missing sstream include

[tvrusso]

src/c_boost/expr/ast_common.hpp makes use of std::istringstream, but does not include <sstream>.

Some boost header must have silently pulled in sstream, but as of boost 1.84 that no longer happens. Trying to build XDM with boost 1.84 fails as a result.

This commit makes ast_common.hpp include <sstream> itself, and that fixes the problem.

Tagging @Karlsefni2012 @gjtempl @TBird2001 directly so they see this sooner.

Closes GH-11

(hint, if you use git format-patch master..fixsstream on this repo, you can get a patch file you can insert into the official repo with git am)