[KillTheDoctor/CMake] Add missing PRIVATE keyword

[Dani Muñoz via llvm-commits]

 While trying to compile llvm/clang on Windows, the following CMake error
occurred. The reason is a missing PUBLIC/PRIVATE/INTERFACE keyword in
target_link_libraries. A fix for it is attached.

CMake Error at utils/KillTheDoctor/CMakeLists.txt:5 (target_link_libraries):
  The keyword signature for target_link_libraries has already been used with
  the target "KillTheDoctor".  All uses of target_link_libraries with a
  target must be either all-keyword or all-plain.

  The uses of the keyword signature are here:

   * cmake/modules/AddLLVM.cmake:771 (target_link_libraries)
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