Mark the Q_DECLARE_OPAQUE_POINTER macro as introduced in 5.0

Change-Id: Ida52c9f1f72b628d9a7faaf7998798e781b53df2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qmetatype.cpp

@@ -90,6 +90,7 @@ struct DefinedTypesFilter {
 /*!
     \macro Q_DECLARE_OPAQUE_POINTER(PointerType)
     \relates QMetaType
+    \since 5.0
 
     This macro enables pointers to forward-declared types (\a PointerType)
     to be registered with QMetaType using either Q_DECLARE_METATYPE()