Fix unsafe comment

[jgfouca]

Having a comment that ends in \ is not portable across different build systems due to the order and manner in which preprocessing is done. In some cases, like when using ninja, the \ can be interpreted as a line continuation which can silently remove the following line of code.

In this case, the line trndir(k+1) = trndir(k)*trnlay(k) was being commented out in the preprocessed file, leading to a very subtle runtime difference in results that took days to track down.

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PR checklist

• Short (1 sentence) summary of your PR:
  Fix unsafe comment with trailing \
• Developer(s):
  @jgfouca
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  Only changes a comment, no testing should be necessary
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please document the changes in detail, including why the changes are made. This will become part of the PR commit log.

[apcraig]

Happy to implement this change, thanks @jgfouca.