Mark PROTOBUF_INCLUDE_DIRS as BUILD_INTERFACE

[okapies]

It fixes a failure when running cmake command if:

1. onnx project is included in other CMake project as a subdirectory and put its outputs to build directory

add_subdirectory(${CMAKE_SOURCE_DIR}/external/onnx
include_directories("${ONNX_INCLUDE_DIRS}")

and

2. protobuf binary is also put in the build directory

because PROTOBUF_INCLUDE_DIRS is not marked as BUILD_INTERFACE in target_include_directories.

$ cmake ..
-- CMAKE_BUILD_TYPE is unset, defaulting to Release
-- Build type: Release
-- Adding external/onnx
-- Adding external/onnx
-- 
-- ******** Summary ********
--   CMake version         : 3.5.1
--   CMake command         : /usr/bin/cmake
--   System                : Linux
--   C++ compiler          : /usr/bin/c++
--   C++ compiler version  : 5.5.0
--   CXX flags             : 
--   Build type            : Release
--   CMAKE_PREFIX_PATH     : 
--   CMAKE_INSTALL_PREFIX  : /usr/local
--   CMAKE_MODULE_PATH     : /home/okapies/myproject/cmake
-- 
--   ONNX version          : 1.2.2
--   ONNX NAMESPACE        : onnx
--   ONNX_BUILD_TESTS      : OFF
--   ONNX_BUILD_BENCHMARKS : OFF
--   ONNX_USE_LITE_PROTO   : OFF
-- 
--   Protobuf compiler     : /home/okapies/myproject/build/protobuf-3.6.1/bin/protoc
--   Protobuf includes     : /home/okapies/myproject/build/protobuf-3.6.1/include
--   Protobuf libraries    : /home/okapies/myproject/build/protobuf-3.6.1/lib/libprotobuf.a
--   BUILD_ONNX_PYTHON     : OFF
-- Adding test

...

-- Adding include
-- Configuring done
CMake Error in external/onnx/CMakeLists.txt:
  Target "onnx_proto" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "/home/okapies/myproject/build/protobuf-3.6.1/include"

  which is prefixed in the build directory.


CMake Error in external/onnx/CMakeLists.txt:
  Target "onnx_proto" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "/home/okapies/myproject/build/protobuf-3.6.1/include"

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

    "/home/okapies/myproject/build/protobuf-3.6.1/include"

  which is prefixed in the source directory.

[CLAassistant]

All committers have signed the CLA.

[zrphercule]

@okapies Hi! Thanks for you contribution!
Would you mind sign the CLA first?

[okapies]

I'm sorry for my late response. I signed your CLA. Could you please check it? Thanks. @zrphercule

[houseroad]

LGTM, thanks