Fix compilation error for default build instructions

[csirianni]

Resolves #1.

The default build instructions include the following command:

cmake -DFAISS_ENABLE_GPU=OFF -DFAISS_ENABLE_PYTHON=OFF -DBUILD_TESTING=ON -DBUILD_SHARED_LIBS=ON -DCMAKE_BUILD_TYPE=Release -B build

which results in the following error:

CMake Error in faiss/CMakeLists.txt:
  Target "faiss" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "/acorn/build/_deps/nlohmann_json-src/include"

  which is prefixed in the build directory.


CMake Error in faiss/CMakeLists.txt:
  Target "faiss" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "/acorn/build/_deps/nlohmann_json-src/include"

  which is prefixed in the build directory.Target "faiss"
  INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "/acorn/build/_deps/nlohmann_json-src/include"

  which is prefixed in the source directory.

The solution, as proposed in #1, is to change the CMakeList.txt dependency
specified in target_include_directories(faiss PUBLIC ${nlohmann_json_SOURCE_DIR}/include) from PUBLIC to PRIVATE.