Update expat to 2.2.0 to fix CVE vulnerability.

third_party/expat/files/lib/expat_external.h.original

 /* Copyright (c) 1998, 1999, 2000 Thai Open Source Software Center Ltd
    See the file COPYING for copying permission.
 */
 
 #ifndef Expat_External_INCLUDED
 #define Expat_External_INCLUDED 1
 
 /* External API definitions */
 
 #if defined(_MSC_EXTENSIONS) && !defined(__BEOS__) && !defined(__CYGWIN__)
@@ skipping 47 matching lines @@
 #ifndef XML_BUILDING_EXPAT
 /* using Expat from an application */
 
 #ifdef XML_USE_MSC_EXTENSIONS
 #define XMLIMPORT __declspec(dllimport)
 #endif
 
 #endif
 #endif  /* not defined XML_STATIC */
 
+#if !defined(XMLIMPORT) && defined(__GNUC__) && (__GNUC__ >= 4)
+#define XMLIMPORT __attribute__ ((visibility ("default")))
+#endif
 
 /* If we didn't define it above, define it away: */
 #ifndef XMLIMPORT
 #define XMLIMPORT
 #endif
 
+#if defined(__GNUC__) && (__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96))
+#define XML_ATTR_MALLOC __attribute__((__malloc__))
+#else
+#define XML_ATTR_MALLOC
+#endif
+
+#if defined(__GNUC__) && ((__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3))
+#define XML_ATTR_ALLOC_SIZE(x)  __attribute__((__alloc_size__(x)))
+#else
+#define XML_ATTR_ALLOC_SIZE(x)
+#endif
 
 #define XMLPARSEAPI(type) XMLIMPORT type XMLCALL
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 #ifdef XML_UNICODE_WCHAR_T
 #define XML_UNICODE
 #endif
@@ skipping 22 matching lines @@
 #else
 typedef long XML_Index;
 typedef unsigned long XML_Size;
 #endif /* XML_LARGE_SIZE */
 
 #ifdef __cplusplus
 }
 #endif
 
 #endif /* not Expat_External_INCLUDED */