Roll expat to 2.5.0

Rolls expat, which was last updated approximately a year ago, to the
very recent 2.5.0 tag.

Fixes flutter/flutter#114734

Author: stuartmorgan-g

DEPS

@@ -415,7 +415,7 @@ deps = {
    Var('chromium_git') + '/external/colorama.git' + '@' + '799604a1041e9b3bc5d2789ecbd7e8db2e18e6b8',
 
   'src/third_party/expat':
-   Var('chromium_git') + '/external/github.com/libexpat/libexpat.git' + '@' + 'a28238bdeebc087071777001245df1876a11f5ee',
+   Var('chromium_git') + '/external/github.com/libexpat/libexpat.git' + '@' + '654d2de0da85662fcc7644a7acd7c2dd2cfb21f0',
 
   'src/third_party/freetype2':
    Var('flutter_git') + '/third_party/freetype2' + '@' + '3bea2761290a1cbe7d8f75c1c5a7ad727f826a66',