interface: clarify error message when adding existing interface

When adding a network interface that is already known to babeld, an
error message is displayed and the new configuration is ignored. This
message is adjusted to include the name of the interface being added so
it is visible from the logs without context

interface.c

@@ -68,8 +68,8 @@ add_interface(char *ifname, struct interface_conf *if_conf)
         if(strcmp(ifp->name, ifname) == 0) {
             if(if_conf)
                 fprintf(stderr,
-                        "Warning: attempting to add existing interface, "
-                        "new configuration ignored.\n");
+                        "Warning: attempting to add existing interface (%s), "
+                        "new configuration ignored.\n", ifname);
             return ifp;
         }
     }