Fix compilation issues on GPU

[hobywan]

It fixes the issues when compiling with NVCC.
There is still a runtime error, unfortunately:

 -------------------------- 
Beam created, charge per particle = -1.2483e+07 (e).
terminate called after throwing an instance of 'std::runtime_error'
  what():  cudaDeviceSynchronize() error( cudaErrorIllegalAddress): an illegal memory access was encountered Kokkos_Cuda_Instance.cpp:144
Traceback functionality not available

[ko044:41696] *** Process received signal ***

I will try to figure out and fix the issue in another pull request.
But in the meantime, I think we could use this version for future updates.