use fabs instead of abs in set_phi to be nice with sycl/clang20

[flg]

This correction was suggested by @niermann999 as a fix for a compile error with a recent SYCL compiler. And worked. This is mostly to remind detray and ACTS experts that this is something to look into. Maybe this needs to be applied elsewhere.

For info the compiler is:

$ clang++ -v
clang version 20.0.0git (https://github.com/intel/llvm e95b34bb2bcef2641d88ca2e09d628039ef269d7)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/intel/oneapi/compiler/2025.0/bin
Build config: +assertions
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/13
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/13
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
Found HIP installation: /opt/rocm, version 6.2.41134

from 24/11/2024 daily release of Intel's LLVM:
https://github.com/intel/llvm/releases/download/nightly-2024-11-24/sycl_linux.tar.gz

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[niermann999]

Looks good