[libclc] [libclc] [cmake] Fix per-target *_convert.cl dependencies (PR #127315)

[Sam James via cfe-commits]

https://github.com/thesamesam approved this pull request.

Please note in the commit message that you're using the same check from `libclc/CMakeLists.txt`. LGTM with that.

https://github.com/llvm/llvm-project/pull/127315