Resolve ambiguity in multiple inheritance of function GetPropertyByLabel in wxPGPage.

In wxPropertyGridPage class derived from wxPropertyGridPageState and wxPropertyGridInterface, function GetPropertyByLabel is inherited from both base classes.
To resolve this ambiguity, function defined in wxPGInterface is introduced in wxPGPage through 'using-declaration'.
(This is a temporary fix and finally GetPropertyByLabel function should be removed from wxPropertyGridPageState.)

See #15251.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76925 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/propgrid/manager.h

@@ -157,6 +157,12 @@ public:
     */
     void SetSplitterPosition( int splitterPos, int col = 0 );
 
+#ifdef WXWIN_COMPATIBILITY_3_0
+    // To avoid ambiguity between functions inherited
+    // from both wxPropertyGridInterface and wxPropertyGridPageState
+    using wxPropertyGridInterface::GetPropertyByLabel;
+#endif // WXWIN_COMPATIBILITY_3_0
+
 protected:
 
     /** Propagate to other pages.