Cleaning up some warnings and enabling -Werror on Intel GPU CI tests

[yasahi-hpc]

Resolves #106.

• Cleaning icpx warnings on Intel GPUs
• Enabling warnings, but cannot use -Werror because there are already many warnings from Kokkos

[cedricchevalier19]

An external Kokkos build may avoid warnings from Kokkos (by using -isystem).

Anyway it is a great improvement.

[yasahi-hpc]

An external Kokkos build may avoid warnings from Kokkos (by using -isystem).

Anyway it is a great improvement.

Thank you for your review.
So we need to modify CMake a little bit?

The easiest way may be to do the build test with -Werror option using the preinstalled Kokkos
I will do that in another PR