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.

debian/control

@@ -44,7 +44,8 @@ Homepage: http://sipwise.com/
 
 Package: ngcp-panel
 Architecture: all
-Depends: libcatalyst-actionrole-acl-perl,
+Depends: apache2,
+         libcatalyst-actionrole-acl-perl,
          libcatalyst-modules-perl,
          libcatalyst-perl (>= 5.90040),
          libcatalyst-view-tt-perl,