toolbox: turn on -Werror

- Deal with some signedness issues
- Deal with some size issues
- Deal with NULL pointer issues
- Deal with some -Wunused issues

Change-Id: I1479dd90d690084491bae3475f2c547833519a57
diff --git a/toolbox/du.c b/toolbox/du.c
index fc7c943..c8beba5 100644
--- a/toolbox/du.c
+++ b/toolbox/du.c
@@ -76,7 +76,7 @@
 	int64_t totalblocks;
 	int ftsoptions, listfiles;
 	int depth;
-	int Hflag, Lflag, aflag, ch, cflag, dflag, gkmflag, nflag, rval, sflag;
+	int Hflag, Lflag, aflag, ch, cflag, dflag, gkmflag, rval, sflag;
 	const char *noargv[2];
 
 	Hflag = Lflag = aflag = cflag = dflag = gkmflag = sflag = 0;