qt.cfg: Add macro Q_DECLARE_PRIVATE (#1510)

Reference:
https://wiki.qt.io/D-Pointer#Q_DECLARE_PRIVATE_and_Q_DECLARE_PUBLIC

Author: versat

cfg/qt.cfg

@@ -1267,6 +1267,7 @@
   <define name="Q_DECL_EXPORT" value=""/>
   <define name="Q_DECL_IMPORT" value=""/>
   <define name="Q_DECLARE_FLAGS(x,y)" value=""/>
+  <define name="Q_DECLARE_PRIVATE(Class)" value="inline Class##Private* d_func() { return reinterpret_cast&lt;Class##Private*&gt;(qGetPtrHelper(d_ptr)); } inline const Class##Private d_func() const { return reinterpret_cast&lt;const Class##Private *&gt;(qGetPtrHelper(d_ptr)); } friend class Class##Private;"/>
   <define name="Q_DISABLE_COPY(C)" value="C(C&amp;);C&amp; operator=(const C&amp;);"/>
   <define name="Q_ENUM(X)" value=""/>
   <define name="Q_ENUMS(X)" value=""/>