Fix cpp CMakefile

[yibin87]

What problem does this PR solve?

Issue Number: close #xxx
In #347, generate-cpp.sh path is changed, the CMakefile under cpp dir should be updated also.
Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch

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[yibin87]

/close