Fixes this error when building with libxml2 2.14
src/xml_pyx.c:203:36: error: expected ')'
  203 | pyxExternalSubsetHandler(void *ctx ATTRIBUTE_UNUSED, const xmlChar *name,
      |                                    ^

Probably has something to do with this commit in libxml2
https://github.com/GNOME/libxml2/commit/208f27f9641a59863ce1f7d4992df77f7eb0ea9d

--- a/src/xmlstar.h
+++ b/src/xmlstar.h
@@ -16,6 +16,8 @@
 #include <libxml/xpathInternals.h>
 #include <libxml/xmlreader.h>
 
+#define ATTRIBUTE_UNUSED __attribute__((unused))
+
 typedef enum { /* EXIT_SUCCESS = 0, EXIT_FAILURE = 1, */
     EXIT_BAD_ARGS = EXIT_FAILURE+1, EXIT_BAD_FILE,
     EXIT_LIB_ERROR, EXIT_INTERNAL_ERROR } exit_status;