block: Make BlockJobTypes const

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/block/commit.c b/block/commit.c
index 553447e..2227fc2 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -173,7 +173,7 @@
     ratelimit_set_speed(&s->limit, speed / BDRV_SECTOR_SIZE, SLICE_TIME);
 }
 
-static BlockJobType commit_job_type = {
+static const BlockJobType commit_job_type = {
     .instance_size = sizeof(CommitBlockJob),
     .job_type      = "commit",
     .set_speed     = commit_set_speed,