Only include expat.h when XMP is enabled. (backport #1972)

[mergify]

This is an automatic backport of pull request #1972 done by Mergify.

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[codecov]

Codecov Report

Merging #1973 (a6f1013) into main (b385f2d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1973   +/-   ##
=======================================
  Coverage   61.28%   61.28%           
=======================================
  Files          96       96           
  Lines       19079    19079           
  Branches     9748     9748           
=======================================
  Hits        11692    11692           
  Misses       5069     5069           
  Partials     2318     2318           

Impacted Files	Coverage Δ
src/xmp.cpp	76.81% <ø> (ø)

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