[nic_simulator] Add missing attribute in the slot (#11006)

What is the motivation for this PR?
As the subject.

How did you do it?
Add packet_filter and drop to the attribute list.

How did you verify/test it?
Run nic_simulator

Any platform specific information?

Author: lolyu

ansible/dualtor/nic_simulator/nic_simulator.py

@@ -183,7 +183,7 @@ def to_string(self):
 class OVSFlow(StrObj):
     """Object to represent an OVS flow."""
 
-    __slots__ = ("in_port", "output_ports", "group", "priority", "_str_prefix")
+    __slots__ = ("in_port", "packet_filter", "output_ports", "group", "priority", "_str_prefix", "drop")
 
     def __init__(self, in_port, packet_filter=None, output_ports=[], group=None, priority=None):
         self.in_port = in_port