test/read_mixer: add missing stdlib.h include

test/read_mixer.cxx

@@ -35,6 +35,7 @@
 #include <assert.h>
 #include <string.h>
 #include <unistd.h>
+#include <stdlib.h>
 
 const struct filter_plugin *
 filter_plugin_by_name(gcc_unused const char *name)