Fix cmake package export.

[Lasall]

Just some minor fixes for cmake package export:

• Fix spelling mistake introduced in 369046f
• Don't export compiler warning flags to interface.

[mlpack-bot]

Thanks for opening your first pull request in this repository! Someone will review it when they have a chance. In the mean time, please be sure that you've handled the following things, to make the review process quicker and easier:

• All code should follow the style guide
• Documentation added for any new functionality
• Tests added for any new functionality
• Tests that are added follow the testing guide
• Headers and license information added to the top of any new code files
• HISTORY.md updated if the changes are big or user-facing
• All CI checks should be passing

Thank you again for your contributions! 👍

[birm]

Looks good; thanks for the fix/improvement! You may want to update HISTORY.md with this fix though.

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

[mlpack-bot]

Hello there! Thanks for your contribution. I see that this is your first contribution to mlpack. If you'd like to add your name to the list of contributors in src/mlpack/core.hpp and COPYRIGHT.txt and you haven't already, please feel free to push a change to this PR---or, if it gets merged before you can, feel free to open another PR.

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[zoq]

@Lasall thanks for putting this together, can you update https://github.com/mlpack/ensmallen/blob/master/HISTORY.md, to reflect the change, the rest looks good to me.

[zoq]

Thanks, looks good to me.