Fix escapes for namespaced classes in gen_stub.php

Fix escapes for namespaced classes in gen_stub.php

Fix the string generated when the `ns\class` is passed to a macro

    #define ESCAPE(x) #x
    // puts(ESCAPE(ns\class));  // warning: unknown escape sequence: \c
    puts(ESCAPE(ns\\class));  // Properly prints ns\class to stdout.