Fixes #5596: Add proper dependency definition to rudder-webapp (el7 comp...

...atibility)

Ticket: http://www.rudder-project.org/redmine/issues/5596

To be merged in master
I don't think that the ```rhel``` macro is defined in RHEL5:

```
[root@centos-5-64 ~]# rpm --eval "%{rhel}"
%{rhel}
```