[llvm] Fix build if ITTAPI_SOURCE_DIR is specified (PR #106924)

[via llvm-commits]

llvmbot wrote:

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>@Zentrik If you'd like this cherry-pick to the 20.0 release branch you can open an issue for that and comment "/cherry-pick f8287f6" (following the process described in https://groups.google.com/g/llvm-dev/c/SfSqz81zwjg).

Error: Command failed due to missing milestone.

https://github.com/llvm/llvm-project/pull/106924