cppcheck improvements. Add const and override where we can

src/zm_eventstream.h

@@ -133,7 +133,7 @@ class EventStream : public StreamBase {
     void setStreamMode( StreamMode p_mode ) {
       mode = p_mode;
     }
-    void runStream();
+    void runStream() override;
     Image *getImage();
   private:
     bool send_file( const char *file_path );

src/zm_ffmpeg_input.h

@@ -23,10 +23,10 @@ class FFmpeg_Input {
     int Close();
     AVFrame *get_frame( int stream_id=-1 );
     AVFrame *get_frame( int stream_id, double at );
-    int get_video_stream_id() {
+    int get_video_stream_id() const {
       return video_stream_id;
     }
-    int get_audio_stream_id() {
+    int get_audio_stream_id() const {
       return audio_stream_id;
     }
 

src/zm_monitorstream.h

@@ -71,7 +71,7 @@ class MonitorStream : public StreamBase {
     bool setStreamStart(int monitor_id) {
       return loadMonitor(monitor_id);
     }
-    void runStream();
+    void runStream() override;
 };
 
 #endif // ZM_MONITORSTREAM_H