Fixed build with recent versions of libxml2 (fixes #305)

libxml2 recently changed a prototype of the xmlStructuredErrorFunc callback,
adding const modifier to the second parameter.

Adding an explicit cast seems to fix the problem (but I will not be surprised, if
this fix will not work equally well on all distros).

Found for a first time and fixed on Arch linux on about 2 Dec 2023 (Arch is rolling
release, so there is no explicit release number)

airscan-xml.c

@@ -124,7 +124,7 @@ xml_rd_parse (xmlDoc **doc, const char *xml_text, size_t xml_len)
         goto DONE;
     }
 
-    ctxt->sax->serror = xml_rd_error_callback;
+    ctxt->sax->serror = (xmlStructuredErrorFunc) xml_rd_error_callback;
 
     /* Parse the document */
     if (xmlCtxtResetPush(ctxt, xml_text, xml_len, NULL, NULL)) {