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

diff --git a/util-linux/mdev.c b/util-linux/mdev.c
index 33dc5d3..a9c1469 100644
--- a/util-linux/mdev.c
+++ b/util-linux/mdev.c
@@ -287,7 +287,7 @@
 	}
 }
 
-int mdev_main(int argc, char **argv);
+int mdev_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int mdev_main(int argc, char **argv)
 {
 	char *action;