Fix compilation with QT_ENABLE_QEXPLICITLYSHAREDDATAPOINTER_STATICCAST

You cannot direct-initialize a pointer variable with a comma
expression. Need to wrap it in an extra set of (). Instead of that,
wrap with {} on the outside, to make it look less like Lisp.

Amends 0ed039fd134fb8025236847295681f13f1775847. On dev, a follow-up
change has already removed the code in question, leaving only 6.8
affected.

Change-Id: I8d03093849d0bd7b868703218b48b5cb9d2a18f4
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/tools/qshareddata.h

@@ -162,7 +162,7 @@ public:
     Q_NODISCARD_CTOR
     QExplicitlySharedDataPointer(const QExplicitlySharedDataPointer<X> &o) noexcept
 #ifdef QT_ENABLE_QEXPLICITLYSHAREDDATAPOINTER_STATICCAST
-        : d(warnIfQExplicitlySharedDataPointerStaticCastMacroDefined(), static_cast<T *>(o.data()))
+        : d{(warnIfQExplicitlySharedDataPointerStaticCastMacroDefined(), static_cast<T *>(o.data()))}
 #else
         : d(o.data())
 #endif