meson: Only require expat when building wayland-scanner
This code is only required for building wayland-scanner so it should be scoped accordingly. libxml-2.0 will only be required if both "scanner" and "dtd_validation" are set to true.
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This code is only required for building wayland-scanner so it should be scoped accordingly. libxml-2.0 will only be required if both "scanner" and "dtd_validation" are set to true.