Move InterfaceList back to set

There are duplicated entries in InterfaceList and causing dbus object
creation failures for
- /xyz/openbmc_project/network/config
- /xyz/openbmc_project/network/config/dhcp

Example Failure,
```
$ /tmp/phosphor-network-manager &
[1] 23394
$ busctl tree xyz.openbmc_project.Network
`-/xyz
  `-/xyz/openbmc_project
    `-/xyz/openbmc_project/network
$ sdeventplus: timeCallback: sd_bus_add_object_vtable: org.freedesktop.DBus.Error.FileExists: File exists
busctl tree xyz.openbmc_project.Network
`-/xyz
  `-/xyz/openbmc_project
    `-/xyz/openbmc_project/network
      |-/xyz/openbmc_project/network/dummy0
      | |-/xyz/openbmc_project/network/dummy0/ipv4
...
```

Tested:
The object is created with no error now.

Signed-off-by: Willy Tu <wltu@google.com>
Change-Id: Iaf91ceee118a4bd32ec8277e0c1372ccab1cb9d9
diff --git a/src/util.cpp b/src/util.cpp
index 59e83bf..c6aedde 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -283,7 +283,7 @@
         {
             continue;
         }
-        interfaces.emplace_back(ifa->ifa_name);
+        interfaces.emplace(ifa->ifa_name);
     }
     return interfaces;
 }