Allow SignalLevelAgent method calls in dbus policy

The SignalLevelAgent API is currently broken by the system bus's
security policy, which blocks iwd's outgoing method call messages. This
patch punches a hole for method calls on the
net.connman.iwd.SignalLevelAgent interface.

src/iwd-dbus.conf

@@ -11,6 +11,7 @@
     <allow own="net.connman.iwd"/>
     <allow send_destination="net.connman.iwd"/>
     <allow send_interface="net.connman.iwd.Agent"/>
+    <allow send_interface="net.connman.iwd.SignalLevelAgent"/>
   </policy>
 
   <policy group="wheel">