Only build XMLValidator when XMP is enabled.

[kevinbackhouse]

This is a follow-up to #1878. As discussed here, XMLValidator should only be built when XMP is enabled.

[codecov]

Codecov Report

Merging #1883 (b96a49b) into main (f9248f9) will not change coverage.
The diff coverage is n/a.

❗ Current head b96a49b differs from pull request most recent head ce02361. Consider uploading reports for the commit ce02361 to get more accurate results

@@           Coverage Diff           @@
##             main    #1883   +/-   ##
=======================================
  Coverage   60.79%   60.79%           
=======================================
  Files          96       96           
  Lines       18962    18962           
  Branches     9516     9516           
=======================================
  Hits        11527    11527           
  Misses       5134     5134           
  Partials     2301     2301           

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

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