f2fs: Fix build error while CONFIG_NLS=m

If CONFIG_F2FS_FS=y but CONFIG_NLS=m, building fails:

fs/f2fs/file.o: In function `f2fs_ioctl':
file.c:(.text+0xb86f): undefined reference to `utf16s_to_utf8s'
file.c:(.text+0xe651): undefined reference to `utf8s_to_utf16s'

Select CONFIG_NLS to fix this.
Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 61a3da4d5ef8 ("f2fs: support FS_IOC_{GET,SET}FSLABEL")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/Kconfig

@@ -2,6 +2,7 @@
 config F2FS_FS
 	tristate "F2FS filesystem support"
 	depends on BLOCK
+	select NLS
 	select CRYPTO
 	select CRYPTO_CRC32
 	select F2FS_FS_XATTR if FS_ENCRYPTION