Add missing getmount import for split keyboards

[krzysztofwolski]

Noticed error from RPi pico serial feed:

>>> Traceback (most recent call last):
  File "code.py", line 17, in <module>
  File "kb.py", line 43, in __init__
NameError: name 'getmount' is not defined

Adding import seems to fix it :)

[JanLunge]

thanks, yes I missed that import there. locally i had this added to my board while testing so i didn't catch that.