Add myself to AUTHORS (#127298)

I made a few contributions.
-
[flutter/flutter](https://github.com/flutter/flutter/issues?q=author%3ALinXunFeng)
-
[flutter/engine](https://github.com/flutter/engine/pulls?q=author%3ALinXunFeng)

AUTHORS

@@ -115,3 +115,4 @@ Bartek Pacia <barpac02@gmail.com>
 Mike Rydstrom <m.rydstrom@gmail.com>
 Harish Anbalagan <warriorharish95668@gmail.com>
 Kim Jiun <kkimj@hanyang.ac.kr>
+LinXunFeng <linxunfeng@yeah.net>