Fix u256 ODR violation

[mborland]

Closes: #1316

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (d2335da) to head (92f3b2a).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #1318   +/-   ##
=======================================
  Coverage     98.9%   98.9%           
=======================================
  Files          276     276           
  Lines        17902   17902           
  Branches      1909    1909           
=======================================
  Hits         17703   17703           
  Misses         199     199           

Files with missing lines	Coverage Δ
include/boost/decimal/detail/u256.hpp	96.4% <ø> (ø)

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