Only include expat.h when XMP is enabled.

[kevinbackhouse]

Fixes: #1971

[kevinbackhouse]

@clanmills: this is a very simple fix to make sure that the build succeeds when XMP is off and expat isn't installed. I think it might be worth including in 0.27.5.

[clanmills]

I agree. Let's put that into v0.27.5.

[codecov]

Codecov Report

Merging #1972 (03cc591) into 0.27-maintenance (d3f7e92) will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           0.27-maintenance    #1972   +/-   ##
=================================================
  Coverage             46.32%   46.32%           
=================================================
  Files                   146      146           
  Lines                 22950    22950           
  Branches              11781    11781           
=================================================
  Hits                  10632    10632           
  Misses                 6702     6702           
  Partials               5616     5616           

Impacted Files	Coverage Δ
src/xmp.cpp	59.67% <ø> (ø)

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[clanmills]

I see this is suffering the macOS -fstack-clash-protection misery. I'll merge it anyway and fix the build misery elsewhere.