LibGC: Preallocate space before dumping GC graph

Speeds up the append_gc_graph function by preallocating space.
This change reduces the time taken to dump the GC graph by 4%
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Libraries/LibGC/Heap.cpp

@@ -119,6 +119,7 @@ public:
             m_all_live_heap_blocks.set(&block);
             return IterationDecision::Continue;
         });
+        m_work_queue.ensure_capacity(roots.size());
 
         for (auto& [root, root_origin] : roots) {
             auto& graph_node = m_graph.ensure(bit_cast<FlatPtr>(root));