z3++.h: No longer include unused sstream.

This makes some code using the C++ API have to include `<sstream>`
if they used the functionality but didn't include it themselves.

src/api/c++/z3++.h

@@ -23,7 +23,6 @@ Notes:
 #include<cassert>
 #include<ostream>
 #include<string>
-#include<sstream>
 #include<memory>
 #include<vector>
 #include<z3.h>

src/api/julia/z3jl.cpp

@@ -1,4 +1,5 @@
 #include "jlcxx/jlcxx.hpp"
+#include <sstream>
 #include "z3++.h"
 
 using namespace z3;