[Coverity] copy instead of move in `memory_pool`

[AlexeySachkov]

See CID 517725 at https://scan.coverity.com/projects/intel-llvm?tab=analysis_settings

llvm/sycl/include/sycl/ext/oneapi/experimental/async_alloc/memory_pool.hpp

Line 93 in f9a7cc8

memory_pool(std::shared_ptr<detail::memory_pool_impl> Impl) : impl(Impl) {}