decoder/ffmpeg: wrap FFmpeg include in "extern C"

Commit ebae25d175 added that #include, but forgot to wrap it in
"extern C", so the linker tried to look up C++ symbols, causing linker
failure.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1582

src/decoder/plugins/FfmpegIo.cxx

@@ -21,10 +21,13 @@
 #define __STDC_CONSTANT_MACROS
 
 #include "FfmpegIo.hxx"
-#include "libavutil/mem.h"
 #include "../DecoderAPI.hxx"
 #include "input/InputStream.hxx"
 
+extern "C" {
+#include <libavutil/mem.h>
+}
+
 AvioStream::~AvioStream()
 {
 	if (io != nullptr) {