Make SYCL device support opt-in for us too

[mborland]

Closes: #1322

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.24%. Comparing base (2697069) to head (abcd08c).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1323   +/-   ##
========================================
  Coverage    95.24%   95.24%           
========================================
  Files          814      814           
  Lines        69309    69309           
========================================
  Hits         66015    66015           
  Misses        3294     3294           

Files with missing lines	Coverage Δ
test/gegenbauer_test.cpp	100.00% <ø> (ø)
test/pow_test.cpp	100.00% <ø> (ø)
test/test_airy.cpp	94.44% <ø> (ø)
test/test_arcsine.cpp	99.17% <ø> (ø)
test/test_bernoulli.cpp	100.00% <ø> (ø)
test/test_bessel_i.cpp	95.00% <ø> (ø)
test/test_bessel_i.hpp	100.00% <ø> (ø)
test/test_bessel_j.cpp	97.36% <ø> (ø)
test/test_bessel_j.hpp	100.00% <ø> (ø)
test/test_bessel_k.cpp	94.11% <ø> (ø)
... and 54 more

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[will-saunders-ukaea]

This seems to fix our downstream compilation issues. Many thanks.