Fix return type of GetAllocationNextPtr

Returning `void* const` doesn't mean anything (produces a warning with -Wignored-qualifiers). Based on usage, it looks like the returned pointer is expected to point at const data, so I've fixed the declaration to reflect that.

Author: brianosman

include/vk_mem_alloc.h

@@ -10928,7 +10928,7 @@ class VmaBlockVector
     uint32_t GetAlgorithm() const { return m_Algorithm; }
     bool HasExplicitBlockSize() const { return m_ExplicitBlockSize; }
     float GetPriority() const { return m_Priority; }
-    void* const GetAllocationNextPtr() const { return m_pMemoryAllocateNext; }
+    const void* GetAllocationNextPtr() const { return m_pMemoryAllocateNext; }
     // To be used only while the m_Mutex is locked. Used during defragmentation.
     size_t GetBlockCount() const { return m_Blocks.size(); }
     // To be used only while the m_Mutex is locked. Used during defragmentation.