Fix broken get_target_property in DepFindProtobuf.cmake

[codeshaunted]

pb_${tgt} cannot be found. Assumed mistake, as all other calls to get_target_property use ${TGT_${tgt}}, changed accordingly.

[mysql-oca-bot]

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[codeshaunted]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at https://oca.opensource.oracle.com/
Please make sure to include your MySQL bug system user (email) in the returned form.
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[mysql-oca-bot]

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