add portaudio for reading microphones (#55)

sherpa-onnx/csrc/microphone.h

@@ -0,0 +1,18 @@
+// sherpa-onnx/csrc/microphone.h
+//
+// Copyright (c)  2022-2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_MICROPHONE_H_
+#define SHERPA_ONNX_CSRC_MICROPHONE_H_
+
+namespace sherpa_onnx {
+
+class Microphone {
+ public:
+  Microphone();
+  ~Microphone();
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_MICROPHONE_H_