Fix missing std headers in singlethread WASM wrapper

The makeCCSinglethreadWrapper() function generates a C++ file that uses
SHARED_ERROR_HANDLERS (which references std::exception and std::string).
The singlethread wrapper only included <emscripten.h> and ../../z3.h,
missing <stdexcept> and <string>. The threaded wrapper gets those
transitively through <thread>, but the singlethread wrapper does not
include <thread>. Add the two missing includes to fix the build.

src/api/js/scripts/make-cc-wrapper.ts

@@ -304,6 +304,9 @@ extern "C" void async_${fn.name}(${paramList}) {
   return `// THIS FILE IS AUTOMATICALLY GENERATED BY ${path.basename(__filename)}
 // DO NOT EDIT IT BY HAND
 
+#include <stdexcept>
+#include <string>
+
 #include <emscripten.h>
 
 #include "../../z3.h"