Fix return type of GetAllocationNextPtr
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.
GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator,GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator,GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator