replace stdint.h with cstdint

The former is deprecated by C++14. The standard says they are the same:

The header defines all types and macros the same as the C standard library
header<stdint.h>.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/decoder/plugins/FfmpegIo.hxx

@@ -24,7 +24,7 @@ extern "C" {
 #include "libavformat/avio.h"
 }
 
-#include <stdint.h>
+#include <cstdint>
 
 class DecoderClient;
 class InputStream;