kivy: fix app not even starting

```
  File "/home/user/venvs/electrum37/lib64/python3.7/site-packages/kivy/lang/builder.py", line 654, in _apply_rule
    child = cls(__no_builder=True)
TypeError: __init__() got an unexpected keyword argument '__no_builder'
```

follow-up c70484455c

electrum/gui/kivy/uix/screens.py

@@ -179,8 +179,8 @@ class SendScreen(CScreen, Logger):
     payment_request = None  # type: Optional[PaymentRequest]
     parsed_URI = None
 
-    def __init__(self):
-        CScreen.__init__(self)
+    def __init__(self, **kwargs):
+        CScreen.__init__(self, **kwargs)
         Logger.__init__(self)
 
     def set_URI(self, text: str):