[llvm] [offload] Add missing build dependency (PR #149326)

[Michael Kruse via llvm-commits]

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@@ -22,5 +22,9 @@ add_tablegen(offload-tblgen OFFLOAD
   RecordTypes.hpp
   )
 
+if(TARGET cxx-headers)
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Meinersbur wrote:

Could you add a comment for what this dependency is for?

https://github.com/llvm/llvm-project/pull/149326