[PATCH 1/1] build/cmake: Fix CMP0023 warning with libffi

[Dan Liew]

> thank you both for looking at this.
> libffi is only  used in one file
> (lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp) and it is never
> part of the llvm interface.

Apologies Jan. I appear to have hijacked your thread trying to
understand some of CMake's new features.

Given that Brad is happy with your patch and that using PRIVATE in
this case is recommended by CMake's documentation (``Generally, a
dependency should be specified in a use of target_link_libraries()
with the PRIVATE keyword if it is used by only the implementation of a
library``) the patch should be fine to commit.

Do you have commit access? If not let me know and I can commit on your behalf.