Mark QtPrivate::StreamStateSaver's ctor as explicit

A QDataStream* is not a faithful representation of a
QtPrivate::StreamStateSaver, so the ctor should be explicit.

Amends 3c93286f08a80b6e1821d7d63d361742b25c6578.

Pick-to: 6.9 6.8 6.5 5.15
Change-Id: I63f8fa19070960bf70c965ffbc270070d10eacd7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qdatastream.h

@@ -269,7 +269,7 @@ class StreamStateSaver
 {
     Q_DISABLE_COPY_MOVE(StreamStateSaver)
 public:
-    inline StreamStateSaver(QDataStream *s) : stream(s), oldStatus(s->status())
+    explicit StreamStateSaver(QDataStream *s) : stream(s), oldStatus(s->status())
     {
         if (!stream->isDeviceTransactionStarted())
             stream->resetStatus();