move libc related stuff out of platform.h

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/util-linux/freeramdisk.c b/util-linux/freeramdisk.c
index bde6afc..6b9d95e 100644
--- a/util-linux/freeramdisk.c
+++ b/util-linux/freeramdisk.c
@@ -8,7 +8,7 @@
  *
  * Licensed under GPLv2, see file LICENSE in this tarball for details.
  */
-
+#include <sys/mount.h>
 #include "libbb.h"
 
 /* From <linux/fd.h> */