fix: include <sstream> in audio cell of demo notebook (#481)

The demo notebook's audio cell uses std::stringstream but does not
include <sstream>. Some toolchains failed to compile the cell as a
result. The wasm demo notebook already includes it.

Closes #480.

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

Author: mvanhorn

test/Notebooks/xeus-cpp.ipynb

@@ -358,6 +358,7 @@
    "source": [
     "#include <string>\n",
     "#include <fstream>\n",
+    "#include <sstream>\n",
     "\n",
     "#include \"nlohmann/json.hpp\"\n",
     "\n",