MAINTAINERS: Add migration/block* to block subsystem

We are moving block-migration.c to the separated migration directory,
keep this file watched by block maintainers is a good idea.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

MAINTAINERS

@@ -757,6 +757,7 @@ F: aio-*.c
 F: block*
 F: block/
 F: hw/block/
+F: migration/block*
 F: qemu-img*
 F: qemu-io*
 F: tests/image-fuzzer/