ncpfs: fix unused variable warning

Without CONFIG_NCPFS_NLS the following warning is seen:

fs/ncpfs/dir.c: In function 'ncp_hash_dentry':
fs/ncpfs/dir.c:136:23: warning: unused variable 'sb' [-Wunused-variable]
   struct super_block *sb = dentry->d_sb;

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>