Update expat to 2.2.0 to fix CVE vulnerability.

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

Index: third_party/expat/files/lib/expat_external.h.original
diff --git a/third_party/expat/files/lib/expat_external.h b/third_party/expat/files/lib/expat_external.h.original
similarity index 87%
copy from third_party/expat/files/lib/expat_external.h
copy to third_party/expat/files/lib/expat_external.h.original
index 2c03284ea265bc62bbae15e29e48f7b0df4cef0d..aa08a2f84c07e4afc3773289da641f71d2c14d40 100644
--- a/third_party/expat/files/lib/expat_external.h
+++ b/third_party/expat/files/lib/expat_external.h.original
@@ -65,12 +65,26 @@
 #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