Fix pessimizing move warning

[mcbarton]

This PR fixes the following warning (found by accident while looking into something else)

xholder.cpp:37:35: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
   37 |         : p_holder(rhs.p_holder ? std::move(rhs.p_holder->clone()) : nullptr)
      |                                   ^

xholder.cpp:37:35: note: remove std::move call here
   37 |         : p_holder(rhs.p_holder ? std::move(rhs.p_holder->clone()) : nullptr)

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.84%. Comparing base (cc947d0) to head (5e4a1c5).

Files with missing lines	Patch %	Lines
src/xholder.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #498   +/-   ##
=======================================
  Coverage   72.84%   72.84%           
=======================================
  Files          18       18           
  Lines        1204     1204           
  Branches      113      113           
=======================================
  Hits          877      877           
  Misses        327      327           

Files with missing lines	Coverage Δ
src/xholder.cpp	64.28% <0.00%> (ø)

Files with missing lines	Coverage Δ
src/xholder.cpp	64.28% <0.00%> (ø)

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[github-actions]

clang-tidy review says "All clean, LGTM! 👍"