fix include error in g++ 4.8

Author: hongweipeng

include/jsoncons/source.hpp

@@ -37,7 +37,7 @@ namespace jsoncons {
             null_buffer(null_buffer&&) = default;
             null_buffer& operator=(null_buffer&&) = default;
 
-            int_type overflow( int_type ch = traits_type::eof() ) override
+            int_type overflow( int_type ch = std::streambuf::traits_type::eof() ) override
             {
                 return ch;
             }