Update to mmtk-core PR #988 (#255)

This PR updates mmtk-core to mmtk/mmtk-core#988.
Once mmtk/mmtk-core#988 is merged, this PR
should be updated to the new mmtk-core commit, and will be auto merged
(if all the requirements for auto merging is met).

---------

Co-authored-by: mmtkgc-bot <[email protected]>

Author: qinsoon

mmtk/Cargo.lock

@@ -32,7 +32,7 @@ memoffset = "0.9.0"
 # - change branch
 # - change repo name
 # But other changes including adding/removing whitespaces in commented lines may break the CI.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "9a676e68dd41272b57e6812dc404ffa6391668e2" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "06a2e5d9773c9f7a32a6afc1add9cef311432dbc" }
 # Uncomment the following to build locally
 # mmtk = { path = "../repos/mmtk-core" }