Fix issue on clang about implicit instantiation of undefined template

Generated code does not include <string>, which is referenced in templates.
Clang may be incorrectly or aggresively instantiating some template.

E.g.,
import_stl_b_wrap.cxx:3199:51: error: implicit instantiation of undefined template 'std::__1::basic_string<char, std::__1::char_traits<char>

Lib/std/std_common.i

@@ -73,7 +73,13 @@ namespace std {
 #endif
 %}
 
-%fragment("StdTraitsCommon","header") %{
+%fragment("StdStringInclude","header") %{
+#ifdef __clang__
+#include <string>
+#endif
+%}
+
+%fragment("StdTraitsCommon","header",fragment="StdStringInclude") %{
 namespace swig {  
   template <class Type>
   struct noconst_traits {