Add missing include of stdlib.h.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/456

src/decoder/plugins/AudiofileDecoderPlugin.cxx

@@ -33,6 +33,7 @@
 
 #include <assert.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 static constexpr Domain audiofile_domain("audiofile");