fix build error with ndk (#253)

sherpa-ncnn/csrc/symbol-table.cc

@@ -23,10 +23,12 @@
 #include <fstream>
 #include <sstream>
 
+
 #if __ANDROID_API__ >= 9
 #include "android/asset_manager.h"
 #include "android/asset_manager_jni.h"
 #include "android/log.h"
+#include <strstream>
 #endif
 
 namespace sherpa_ncnn {