fix install command for menoh_onnx and menoh_onnx_proto

[okdshin]

Modification for #215

[msakai]

This ended up with the following error:

$ cmake .. -DBUILD_SHARED_LIBS=OFF -DENABLE_TEST=OFF -DENABLE_EXAMPLE=ON
-- The C compiler identification is AppleClang 10.0.0.10001044
-- The CXX compiler identification is AppleClang 10.0.0.10001044
-- Check for working C compiler: /Library/Developer/CommandLineTools/usr/bin/cc
-- Check for working C compiler: /Library/Developer/CommandLineTools/usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /Library/Developer/CommandLineTools/usr/bin/c++
-- Check for working CXX compiler: /Library/Developer/CommandLineTools/usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- CMAKE_BUILD_TYPE is unset, defaulting to Release
-- Build type: Release
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - found
-- Found Threads: TRUE  
-- Found Protobuf: /usr/local/lib/libprotobuf.dylib (found version "3.6.1") 
-- Found Git: /usr/bin/git (found version "2.17.2 (Apple Git-113)") 
-- Adding external/onnx
fatal: /Users/sakai/menoh/external/onnx: '/Users/sakai/menoh/external/onnx' is outside repository
-- 
-- ******** Summary ********
--   CMake version         : 3.12.4
--   CMake command         : /usr/local/Cellar/cmake/3.12.4/bin/cmake
--   System                : Darwin
--   C++ compiler          : /Library/Developer/CommandLineTools/usr/bin/c++
--   C++ compiler version  : 10.0.0.10001044
--   CXX flags             :  -Wnon-virtual-dtor
--   Build type            : Release
--   Compile definitions   : 
--   CMAKE_PREFIX_PATH     : 
--   CMAKE_INSTALL_PREFIX  : /usr/local
--   CMAKE_MODULE_PATH     : /Users/sakai/menoh/cmake
-- 
--   ONNX version          : 1.4.1
--   ONNX NAMESPACE        : menoh_onnx
--   ONNX_BUILD_TESTS      : OFF
--   ONNX_BUILD_BENCHMARKS : OFF
--   ONNX_USE_LITE_PROTO   : OFF
--   ONNXIFI_DUMMY_BACKEND : OFF
-- 
--   Protobuf compiler     : /usr/local/bin/protoc
--   Protobuf includes     : /usr/local/include
--   Protobuf libraries    : /usr/local/lib/libprotobuf.dylib
--   BUILD_ONNX_PYTHON     : OFF
CMake Error at CMakeLists.txt:111 (install):
  install TARGETS given target "onnx_proto" which does not exist in this
  directory.


-- Adding benchmark
-- Adding example
-- Found OpenCV: /usr/local (found version "3.4.3") 
-- Adding menoh
-- Found MKLDNN: /usr/local/lib/libmkldnn.dylib (Required is at least version "0.14") 
-- Adding include
-- Configuring incomplete, errors occurred!
See also "/Users/sakai/menoh/build/CMakeFiles/CMakeOutput.log".