Fix Kokkos kernels for Kokkos 4.3.0

[caschb]

Kokkos kernels don't compile under the latest release. Math functions such as fabs have been moved from the Kokkos::Experimental namespace to the Kokkos namespace. Additionally, directly including headers aside from Kokkos_Core.hpp is disallowed.

https://github.com/ParRes/Kernels/issues/640

Do you certify that your contribution is made in good faith and does not attempt to introduce any negative behavior into this project?

• [X] Yes
• [ ] No

[jeffhammond]

It's too bad Travis CI went away and regressions aren't caught anymore. I haven't had time to do GitHub CI yet.

Anyways, I'll verify the change and merge soon.