[roottest][io] Bring back __CLING__ guard in Hard2Stream members

Follows up on 5727e1d, bringing back a `__CINT__` preprocessor guard
that should have stayed and rename it to `__CLING__`.

Author: guitargeek

roottest/root/io/customStreamer/header.h

@@ -2,7 +2,9 @@
 #include "TClass.h"
 class Hard2Stream {
 private:
+#ifndef __CLING__
    double val;
+#endif
 public:
    Hard2Stream() : val(-1) {};
    Hard2Stream(double v) : val(v) {};