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

diff --git a/coreutils/df.c b/coreutils/df.c
index 6fe8e2f..ad6a4f3 100644
--- a/coreutils/df.c
+++ b/coreutils/df.c
@@ -29,7 +29,7 @@
 }
 #endif
 
-int df_main(int argc, char **argv);
+int df_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int df_main(int argc, char **argv)
 {
 	unsigned long blocks_used;