MAINTAINERS: Add Claudio as s390x maintainer

Claudio has added his own tests, reviewed the tests of others and
added to the common as well as the s390x library with excellent
results. So it's time to make him a s390x maintainer.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20211021145912.79225-1-frankja@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>