Fixed some warnings in tensor runtime

...s/hpvm-tensor-rt/tensor_runtime/include/image/stb_image.h

...ects/hpvm-tensor-rt/tensor_runtime/include/op_overheads.h

...ts/hpvm-tensor-rt/tensor_runtime/src/approx_simulation.cu

llvm/projects/hpvm-tensor-rt/tensor_runtime/src/error.cu

...ts/hpvm-tensor-rt/tensor_runtime/src/img_tensor_utils.cpp

...rojects/hpvm-tensor-rt/tensor_runtime/src/op_overheads.cc

...jects/hpvm-tensor-rt/tensor_runtime/src/tensor_runtime.cu