Add #pragma once to providers.h, so avoid 'struct' redefinition error when including the header from multiple places.
Add #pragma once to providers.h, so avoid 'struct' redefinition error when including the header from multiple places.

microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime,microsoft/onnxruntime