QMetaTypeInterface: Do not make member itself const

Dynamic metatypes, like the ones used in QML any Python, benefit if the
pointer in the interface is mutable: They can -either by inheritance
from QMetaTypeInterface or by copying one of its instances- get a
"prefilled" interface, and then replace any member to fit their use
case. Except that before this change, this was not possible for
metaObjectFn.

Change-Id: I6afba13f97174eb08391af38b73fcfc7b37a2276
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qmetatype.h

@@ -268,7 +268,7 @@ public:
     mutable QBasicAtomicInt typeId;
 
     using MetaObjectFn = const QMetaObject *(*)(const QMetaTypeInterface *);
-    const MetaObjectFn metaObjectFn;
+    MetaObjectFn metaObjectFn;
 
     const char *name;