merkle-tree: fix build when targeting bpf (bp #16335)

[mergify]

This is an automatic backport of pull request #16335 done by Mergify.

Cherry-pick of ddc0a16 has failed:

On branch mergify/bp/v1.6/pr-16335
Your branch is ahead of 'origin/v1.6' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit ddc0a16ce.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   Cargo.lock
	modified:   merkle-tree/src/merkle_tree.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   merkle-tree/Cargo.toml

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[mergify]

automerge label removed due to a CI failure

[mergify]

automerge label removed due to a CI failure

[mergify]

automerge label removed due to a CI failure

[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #16342 (de9326b) into v1.6 (6184254) will increase coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             v1.6   #16342   +/-   ##
=======================================
  Coverage    80.0%    80.1%           
=======================================
  Files         410      410           
  Lines      109413   109413           
=======================================
+ Hits        87625    87647   +22     
+ Misses      21788    21766   -22