Fix build with the current FFmpeg avformat (#190)

* signature for function libavformat/avio.h avio_alloc_context, parameter write_packet has changed from
(void *opaque, uint8_t *buf, int buf_size) to (void *opaque, const uint8_t *buf, int buf_size)
This was previously deprecated, but removed on March 07, 2024.
Please see FFmpeg/FFmpeg@02aea61
for details.
This commit reflects that change.

* Update src/libav_streamer.cpp

---------

Co-authored-by: Błażej Sowa <bsowa123@gmail.com>

Author: jfrancis71

src/libav_streamer.cpp

@@ -77,7 +77,11 @@ LibavStreamer::~LibavStreamer()
 }
 
 // output callback for ffmpeg IO context
+#if LIBAVFORMAT_VERSION_MAJOR < 61
 static int dispatch_output_packet(void * opaque, uint8_t * buffer, int buffer_size)
+#else
+static int dispatch_output_packet(void * opaque, const uint8_t * buffer, int buffer_size)
+#endif
 {
   async_web_server_cpp::HttpConnectionPtr connection =
     *((async_web_server_cpp::HttpConnectionPtr *) opaque);