[QNN-EP] Minor fix: compare different types (#24550)
Fix:
```/local/mnt/workspace/onnxruntime-qnn-ep/onnxruntime/core/providers/qnn/builder/opbuilder/softmax_op_builder.cc:
In function ‘std::vector<unsigned int>
onnxruntime::qnn::FlattenShapeFromAxis(std::vector<unsigned int>&,
int32_t)’:
/local/mnt/workspace/onnxruntime-qnn-ep/onnxruntime/core/providers/qnn/builder/opbuilder/softmax_op_builder.cc:47:28:
error: comparison of integer expressions of different signedness:
‘int32_t’ {aka ‘int’} and ‘std::vector<unsigned int>::size_type’ {aka
‘long unsigned int’} [-Werror=sign-compare]
47 | assert(axis >= 0 && axis < input_shape.size());
|```
