MAINTAINERS: add myself for write after approval

ChangeLog:

2020-11-13  Haochen Gui  <guihaoc@gcc.gnu.org>

	* MAINTAINERS (Write After Approval): add myself

MAINTAINERS

@@ -409,6 +409,7 @@ Matthew Gretton-Dann				<matthew.gretton-dann@arm.com>
 Yury Gribov					<tetra2005@gmail.com>
 Jon Grimm					<jgrimm2@us.ibm.com>
 Laurent Guerby					<laurent@guerby.net>
+Haochen Gui					<guihaoc@gcc.gnu.org>
 Jiufu Guo					<guojiufu@linux.ibm.com>
 Xuepeng Guo					<terry.xpguo@gmail.com>
 Wei Guozhi					<carrot@google.com>