exfat-tools: add device fd close

Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
diff --git a/mkfs/mkfs.c b/mkfs/mkfs.c
index c737521..d5409ad 100644
--- a/mkfs/mkfs.c
+++ b/mkfs/mkfs.c
@@ -654,5 +654,6 @@
 
 	fsync(bd.dev_fd);
 out:
+	close(bd.dev_fd);
 	return ret;
 }