QRingBuffer: make constructor explicit

QRingBuffer: make constructor explicit

This is a private class, but it's so close to
the classical Stack(int) example for explicit
that I just have to make this ctor explicit,
too:

   QRingBuffer rb = 0; // oops: meant '*rb'

now no longer compiles.

Change-Id: I7d58c1f08c1b14d14930426159c5c8db71b4cf4d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>