suppress warnings about easch <applet>_main() having

no preceding prototype

docs/new-applet-HOWTO.txt

@@ -35,6 +35,7 @@ For a new applet mu, here is the code that would go in mu.c:
 #include "busybox.h"
 #include <other.h>
 
+int mu_main(int argc, char **argv);
 int mu_main(int argc, char **argv)
 {
 	int fd;