fix cmake parse path string error (#88)

Co-authored-by: Jian You (jianyou) <jianyou@cisco.com>

cmake/onnxruntime.cmake

@@ -100,6 +100,7 @@ function(download_onnxruntime)
   foreach(f IN LISTS possible_file_locations)
     if(EXISTS ${f})
       set(onnxruntime_URL  "${f}")
+      file(TO_CMAKE_PATH "${onnxruntime_URL}" onnxruntime_URL)
       set(onnxruntime_URL2)
       break()
     endif()