Fix build with wxNO_RTTI

Since CreateFromUnknownDC() can return NULL anyway, just always define it,
and have it always return NULL with wxNO_RTTI
See #17765

include/wx/graphics.h

@@ -438,10 +438,8 @@ public:
 #endif
 #endif
 
-#ifndef wxNO_RTTI
     // Create a context from a DC of unknown type, if supported, returns NULL otherwise
     static wxGraphicsContext* CreateFromUnknownDC(const wxDC& dc);
-#endif
 
     static wxGraphicsContext* CreateFromNative( void * context );