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.

examples/c++/example.cpp

@@ -5,6 +5,7 @@ Copyright (c) 2015 Microsoft Corporation
 --*/
 
 #include <iostream>
+#include <sstream>
 #include<vector>
 #include"z3++.h"
 

examples/tptp/tptp5.cpp

@@ -15,6 +15,7 @@ Copyright (c) 2015 Microsoft Corporation
 #include <iostream>
 #include <fstream>
 #include <limits>
+#include <sstream>
 #include <string.h>
 #include <cstdlib>
 #include "z3++.h"