Changed virtual override to final

It's possible that someone might try to derive from and override
here, and if so the result will be perhaps not as expected. This
change will make it slightly harder to produce an unexpected
call to the wrong vtable dispatch from the ctor.

Signed-off-by: Charlie Vigue <charlie.vigue@openvpn.com>

openvpn/buffer/bufcomposed.hpp

@@ -59,7 +59,7 @@ class BufferComposed
             return iter != bc.bv.end();
         }
 
-        virtual void next_buffer() override
+        virtual void next_buffer() final
         {
             if (iter_defined())
                 reset_buf(**iter++);