auto-t: fix SignalAgent Release arguments

The path for IWD to call this doesn't ever happen in autotests
but during debugging of the DPP agent it was noticed that the
DBus signature was incorrect and would always result in an error
when calling from IWD.

autotests/util/iwd.py

@@ -201,9 +201,9 @@ class SignalAgent(dbus.service.Object):
         return self._path
 
     @dbus.service.method(IWD_SIGNAL_AGENT_INTERFACE,
-                         in_signature='', out_signature='')
-    def Release(self):
-        print("SignalAgent released")
+                         in_signature='o', out_signature='')
+    def Release(self, path):
+        print("SignalAgent released %s" % path)
 
     @dbus.service.method(IWD_SIGNAL_AGENT_INTERFACE,
                          in_signature='oy', out_signature='')