isofs: fix build regression

The new isofs_show_options() function fails to build when CONFIG_NLS
is disabled:

fs/isofs/inode.c: In function 'isofs_show_options':
fs/isofs/inode.c:518:44: error: 'CONFIG_NLS_DEFAULT' undeclared (first use in this function)
fs/isofs/inode.c:518:44: note: each undeclared identifier is reported only once for each function it appears in

This adds a check for CONFIG_JOLIET (which selects NLS), matching
the other uses of the iocharset handling in this file.

Fixes: 6fecb86a44f5 ("isofs: Implement show_options")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jan Kara <jack@suse.cz>