Add apache2 to Depends

Addressing:

| 17:43:21   /var/lib/dpkg/info/ngcp-panel.postinst: 19: /var/lib/dpkg/info/ngcp-panel.postinst: a2enmod: not found
| 17:43:21   dpkg: error processing ngcp-panel (--configure):
| 17:43:21    subprocess installed post-installation script returned error exit status 127

  -- https://jenkins.mgm.sipwise.com/view/piuparts/job/ngcp-panel-piuparts/323/console

We use a2enmod in the postinst maintainer script, therefore
depending on the apache2 package.