target-s390: Claim maintainership

Signed-off-by: Richard Henderson <rth@twiddle.net>
diff --git a/MAINTAINERS b/MAINTAINERS
index cf39e5a..4b7553e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -98,6 +98,7 @@
 F: target-ppc/
 
 S390
+M: Richard Henderson <rth@twiddle.net>
 M: Alexander Graf <agraf@suse.de>
 S: Maintained
 F: target-s390x/