MAINTAINERS: add self as reviewer for sched_ext

Add myself as a reviewer for sched_ext, as I am actively working on this
project and would like to help review relevant patches and address any
related kernel issues.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

MAINTAINERS

@@ -20902,6 +20902,7 @@ F:	kernel/sched/
 SCHEDULER - SCHED_EXT
 R:	Tejun Heo <tj@kernel.org>
 R:	David Vernet <void@manifault.com>
+R:	Andrea Righi <arighi@nvidia.com>
 L:	linux-kernel@vger.kernel.org
 S:	Maintained
 W:	https://github.com/sched-ext/scx