btt/devs: silence warning on sprintf overflow

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/btt/devs.c b/btt/devs.c
index ccaae87..12ce2ca 100644
--- a/btt/devs.c
+++ b/btt/devs.c
@@ -108,7 +108,7 @@
 static inline FILE *open_pit(struct d_info *dip)
 {
 	FILE *fp;
-	char str[256];
+	char str[272];
 
 	sprintf(str, "%s_pit.dat", dip->dip_name);
 	if ((fp = my_fopen(str, "w")) == NULL)