Add cast to test 'string_simple' to avoid C++ compiler error.

Examples/test-suite/string_simple.i

@@ -6,7 +6,7 @@
 #include <string.h>
 const char* copy_string(const char* str) {
   size_t len = strlen(str);
-  char* newstring = malloc(len + 1);
+  char* newstring = (char*) malloc(len + 1);
   strcpy(newstring, str);
   return newstring;
 }