Fix build with musl
https://bugs.gentoo.org/831494

--- a/src/sound_stuff.c
+++ b/src/sound_stuff.c
@@ -33,7 +33,7 @@
 #include <stdlib.h>
 
 #ifndef USE_SDL
-#include <sys/unistd.h>
+#include <unistd.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/ioctl.h>