MAINTAINERS: update BPF JIT S390 maintainers

Ilya Leoshkevich is joining as s390 bpf maintainer. With his background
as gcc developer he would be valuable for the team and community as a
whole. Ilya, have fun!

Since there is now enough eyes on s390 bpf, relieve Christian Borntraeger,
so that he could focus on his maintainer tasks for other components.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@e.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>