[packman] another mythweb packaging error

[packman at howorth.org.uk]

After I installed mythweb and we sorted the PHP version issue, I could
get it to run but it only displayed blank pages. If I looked in 
/var/log/apache2/error_log I saw:

[Wed Jul 25 11:25:53.352956 2018] [php7:error] [pid 7905] [client
127.0.0.1:40748] PHP Fatal error:  Uncaught Error: Class 'MythBackend'
not found in /srv/www/htdocs/mythweb/includes/utils.php:56\nStack
trace:\n#0 /srv/www/htdocs/mythweb/includes/db_update.php(57):
setting('recommend_enabl...', NULL,
false)\n#1 /srv/www/htdocs/mythweb/includes/init.php(46):
require_once('/srv/www/htdocs...')\n#2 /srv/www/htdocs/mythweb/mythweb.php(20):
require_once('/srv/www/htdocs...')\n#3 {main}\n  thrown
in /srv/www/htdocs/mythweb/includes/utils.php on line 56

It tuns out this is due to a missing package. After installing
php-mythtv, mythweb runs.

So I believe that php-mythtv should be a dependency of mythweb.