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>

src/corelib/tools/qringbuffer_p.h

@@ -61,7 +61,7 @@ QT_BEGIN_NAMESPACE
 class QRingBuffer
 {
 public:
-    inline QRingBuffer(int growth = 4096) : basicBlockSize(growth) {
+    explicit inline QRingBuffer(int growth = 4096) : basicBlockSize(growth) {
         buffers << QByteArray();
         clear();
     }