Make Stream's destructor public. (#183)

A public destructor is needed to move around streams with their
interface pointers (or std::unique_ptr<Stream>) instead of the specific
implementation.

include/ozz/base/io/stream.h

@@ -74,12 +74,12 @@ class OZZ_BASE_DLL Stream {
   // Returns the current size of the stream.
   virtual size_t Size() const = 0;
 
- protected:
-  Stream() {}
-
   // Required virtual destructor.
   virtual ~Stream() {}
 
+ protected:
+  Stream() {}
+
  private:
   Stream(const Stream&);
   void operator=(const Stream&);