Modify redefinition of strdup

If the glibc version < 2.26.There are two
definitions of strdup in harness.h and string2.h.
This will cause a redefined error when compile.
Using macro #undef to undef the strdup defined
in string2.h:1291.

harness.h

@@ -63,6 +63,9 @@ void trigger_exception(char *msg);
 /* Tested program use our versions of malloc and free */
 #define malloc test_malloc
 #define free test_free
+
+/* Use undef to avoid strdup redefined error */
+#undef strdup
 #define strdup test_strdup
 
 #endif