Add <string> fragment

Removes <string> include specifically for clang
Cuts down on duplicate #include <string> in general

Lib/std/std_common.i

@@ -23,8 +23,8 @@
 // Common code for supporting the C++ std namespace
 //
 
+%fragment("<string>");
 %{
-#include <string>
 #include <stdexcept>
 #include <stddef.h>
 %}
@@ -73,13 +73,7 @@ namespace std {
 #endif
 %}
 
-%fragment("StdStringInclude","header") %{
-#ifdef __clang__
-#include <string>
-#endif
-%}
-
-%fragment("StdTraitsCommon","header",fragment="StdStringInclude") %{
+%fragment("StdTraitsCommon","header",fragment="<string>") %{
 namespace swig {  
   template <class Type>
   struct noconst_traits {