hotfix saving restart files using GPUs

[ranocha]

It looks like we were not careful enough in #3025, since examples/p4est_3d_dgsem/elixir_advection_basic.jl has a restart callback but examples/p4est_3d_dgsem/elixir_advection_basic_gpu.jl did not. This should at least fix the immediate error (without implementing a nicer interface for loading restart files on GPUs, which is tracked in #2822).

[github-actions]

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[MarcoArtiano]

Thanks! That looks good to me.

Just a question as I'm not familiar with the restart functionality in Trixi: this will not work if someone wants to restart with the GPU?

[ranocha]

Restarts will work, but people need to ensure manually that everything is on the GPU as they want it to be.