Unexpected warning message

[jwiegley]

When I have a module that uses LANGUAGE ForeignFunctionInterface, I get this warning:

Illegal foreign declaration: requires via-C, llvm (-fllvm) or native code generation (-fvia-C)
    When checking declaration:
      foreign export ccall "odbGitHubBackendFreeCallback" odbGitHubBackendFreeCallback
        :: F'git_odb_backend_free_callback

This doesn't happen with ghc or ghc-mod.