Restore resetting of wxMiniFrame cached decor size

This was inadvertently lost in 8cdd20667f, apparently causing wxRESIZE_BORDER
functionality to stop working on some platforms. See #18591

src/gtk/minifram.cpp

@@ -307,6 +307,9 @@ bool wxMiniFrame::Create( wxWindow *parent, wxWindowID id, const wxString &title
     if (style & wxRESIZE_BORDER)
        m_gdkFunc |= GDK_FUNC_RESIZE;
     gtk_window_set_default_size(GTK_WINDOW(m_widget), m_width, m_height);
+
+    // Reset m_decorSize, wxMiniFrame manages its own decorations
+    memset(&m_decorSize, 0, sizeof(m_decorSize));
     m_deferShow = false;
 
     if (m_parent && (GTK_IS_WINDOW(m_parent->m_widget)))