kokkos: fix build failng with KOKKOS_DEFAULT=OFF

[mzuzek]

Fix build failing on missing kokkos_exec when LINALG_ENABLE_KOKKOS is ON but LINALG_ENABLE_KOKKOS_DEFAULT is OFF

[mhoemmen]

Regarding my comment on PR #220: Do you think the failing test could be fixed straightforwardly for now, even if the fix is not the most efficient? I would rather have slow code for now, than a disabled test.

[mzuzek]

@mhoemmen

Regarding my comment on PR #220: Do you think the failing test could be fixed straightforwardly for now, even if the fix is not the most efficient? I would rather have slow code for now, than a disabled test.

Yes, please check #220 (comment).

This fix shall just prevent LINALG_ENABLE_KOKKOS_DEFAULT=OFF builds from failing. On #213 I propose to keep this flag enabled, so that we gain more coverage by also testing Kokkos implementations with native tests too (in Kokkos CI jobs).

[crtrott]

I checked that if DEFAULT is OFF it still does not replace std::par etc.