meson: Fix build when libpng is not found
trace-to-xml.c needs cairo-xml.h which is only available when CAIRO_HAS_XML_SURFACE feature is built which is only available when both zlib and png are found. CAIRO_HAS_INTERPRETER only requires zlib.
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trace-to-xml.c needs cairo-xml.h which is only available when CAIRO_HAS_XML_SURFACE feature is built which is only available when both zlib and png are found. CAIRO_HAS_INTERPRETER only requires zlib.