Fix broken imports

[abatchy17]

Plugin doesn't load in v1.1.1142 due to missing import. #13 is possibly related.

Traceback (most recent call last):
  File "C:\Users\xxx\AppData\Roaming\Binary Ninja\plugins\binja-avr\__init__.py", line 1191, in <module>
    BinaryViewType['ELF'].register_arch(83, Endianness.LittleEndian, Architecture["AVR"])
NameError: name 'BinaryViewType' is not defined
Python plugin 'binja-avr' could not be loaded
Traceback (most recent call last):
  File "C:\Users\xxx\AppData\Roaming\Binary Ninja\plugins\binja-avr\__init__.py", line 1191, in <module>
    BinaryViewType['ELF'].register_arch(83, Endianness.LittleEndian, Architecture["AVR"])
NameError: name 'BinaryViewType' is not defined
Python plugin 'binja-avr' could not be loaded