LibWeb: Put PaintableWithLines in dedicated GC allocator

Author: https://github.com/awesomekling
Commit: c394344e7d
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4424
Reviewed-by: https://github.com/gmta

Libraries/LibWeb/Painting/PaintableBox.h

@@ -317,6 +317,7 @@ private:
 
 class PaintableWithLines : public PaintableBox {
     GC_CELL(PaintableWithLines, PaintableBox);
+    GC_DECLARE_ALLOCATOR(PaintableWithLines);
 
 public:
     static GC::Ref<PaintableWithLines> create(Layout::BlockContainer const&);