Include string.h for memory operations

src/sound/audio4.c

@@ -18,6 +18,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <sys/ioctl.h>
+#include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <math.h>