kern: implement more of KMemoryManager through KPageBuffer slab init

libraries/libmesosphere/include/mesosphere/init/kern_init_slab_setup.hpp

@@ -40,6 +40,7 @@ namespace ams::kern::init {
     const KSlabResourceCounts &GetSlabResourceCounts();
 
     size_t CalculateTotalSlabHeapSize();
+    NOINLINE void InitializeKPageBufferSlabHeap();
     NOINLINE void InitializeSlabHeaps();
 
 }