Fix build error

lavf_device.cpp: In member function 'virtual int lavf_device::start()':
lavf_device.cpp:102:49: error: invalid conversion from 'const AVInputFormat*' to 'AVInputFormat*' [-fpermissive]
  102 |                 input_fmt = av_find_input_format("mjpeg");
      |                             ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
      |                                                 |
      |                                                 const AVInputFormat*

lib/lavf_device.cpp

@@ -77,7 +77,7 @@ int lavf_device::start()
 	if (ctx) return 0;
 
 	AVDictionary *avopt_open_input = NULL;
-	AVInputFormat *input_fmt = NULL;
+	const AVInputFormat *input_fmt = NULL;
 
 	bc_log(Debug, "Opening session from URL: %s", url);