serialhdl: Add a get_serialqueue() method

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/serialhdl.py

@@ -226,6 +226,8 @@ class SerialReader:
         return self.reactor
     def get_msgparser(self):
         return self.msgparser
+    def get_serialqueue(self):
+        return self.serialqueue
     def get_default_command_queue(self):
         return self.default_cmd_queue
     # Serial response callbacks