Fix compiler warning in the xml module (GH-26245)

Fix compiler warning in the xml module (GH-26245)

The newest version of gcc complains about passing un-initialized arrays
as constant pointers:

```
/Modules/expat/xmltok_ns.c: In function ‘findEncodingNS’:
/Modules/expat/xmltok.h:272:10: warning: ‘buf’ may be used uninitialized [-Wmaybe-uninitialized]
  272 |   (((enc)->utf8Convert)(enc, fromP, fromLim, toP, toLim))
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Modules/expat/xmltok_ns.c:95:3: note: in expansion of macro ‘XmlUtf8Convert’
   95 |   XmlUtf8Convert(enc, &ptr, end, &p, p + ENCODING_MAX - 1);
      |   ^~~~~~~~~~~~~~
/Modules/expat/xmltok.h:272:10: note: by argument 5 of type ‘const char *’ to ‘enum XML_Convert_Result(const ENCODING *, const char **, const char *, char **, const char *)’ {aka ‘enum XML_Convert_Result(const struct encoding *, const char **, const char *, char **, const char *)’}
  272 |   (((enc)->utf8Convert)(enc, fromP, fromLim, toP, toLim))
      |   ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Modules/expat/xmltok_ns.c:95:3: note: in expansion of macro ‘XmlUtf8Convert’
   95 |   XmlUtf8Convert(enc, &ptr, end, &p, p + ENCODING_MAX - 1);
      |   ^~~~~~~~~~~~~~
In file included from /Modules/expat/xmltok.c:1657:
/Modules/expat/xmltok_ns.c:92:8: note: ‘buf’ declared here
   92 |   char buf[ENCODING_MAX];

```