add -fvisibility=hidden to CC flags, mark XXX_main functions
EXTERNALLY_VISIBLE. 5% size reduction of libbusybox.so

diff --git a/util-linux/swaponoff.c b/util-linux/swaponoff.c
index db43aa5..6858d26 100644
--- a/util-linux/swaponoff.c
+++ b/util-linux/swaponoff.c
@@ -58,7 +58,7 @@
 	return err;
 }
 
-int swap_on_off_main(int argc, char **argv);
+int swap_on_off_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int swap_on_off_main(int argc, char **argv)
 {
 	int ret;