Fix compiler warning from uninitialized members

When building tst_qmetatype.cpp, clang generates the warning that

class 'AlignmentDummy' does not declare any constructor to initialize
its non-modifiable members

Turn the class into a struct, which doesn't generate such warnings.

Change-Id: I61013a10418238a11824b18ff1e927bbafa46ec2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qarraydata.h

@@ -206,7 +206,7 @@ struct QTypedArrayData
         inline operator const T*() const { return i; }
     };
 
-    class AlignmentDummy { QArrayData header; T data; };
+    struct AlignmentDummy { QArrayData header; T data; };
 
     [[nodiscard]] static QPair<QTypedArrayData *, T *> allocate(qsizetype capacity,
             ArrayOptions options = DefaultAllocationFlags)