nextcloud: add missing php7-mod-filter dependency

nextcloud needs php-mod-filter dependency, otherwise running nextcloud_install fails:

+ sudo -u nobody php-cli ./occ maintenance:install '--database=mysql' '--database-name=nextcloud' '--database-user=nextcloud' '--admin-user=michal' '--admin-pass=1lvKANXvKBoQ3SVTEZCw7Oi5q' '--database-pass=Cwh3LFwwVRd7jUPbz18kRbT' '--database-host=127.0.0.1' '--database-port=3306' --no-interaction
An unhandled exception has been thrown:
Error: Call to undefined function OC\filter_var() in /srv/www/nextcloud/lib/private/Setup.php:479
Stack trace:
#0 /srv/www/nextcloud/lib/private/Setup.php(500): OC\Setup::findWebRoot(Object(OC\SystemConfig))
#1 /srv/www/nextcloud/lib/private/Setup.php(422): OC\Setup::updateHtaccess()
#2 /srv/www/nextcloud/core/Command/Maintenance/Install.php(106): OC\Setup->install(Array)
#3 /srv/www/nextcloud/3rdparty/symfony/console/Command/Command.php(255): OC\Core\Command\Maintenance\Install->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#4 /srv/www/nextcloud/3rdparty/symfony/console/Application.php(1000): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#5 /srv/www/nextcloud/3rdparty/symfony/console/Application.php(271): Symfony\Component\Console\Application->doRunCommand(Object(OC\Core\Command\Maintenance\Install), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#6 /srv/www/nextcloud/3rdparty/symfony/console/Application.php(147): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#7 /srv/www/nextcloud/lib/private/Console/Application.php(215): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#8 /srv/www/nextcloud/console.php(100): OC\Console\Application->run()
#9 /srv/www/nextcloud/occ(11): require_once('/srv/www/nextcl...')
#10 {main}+ die 'Installation failed'