Fix _filter_duplicates so pykg-config actually works.
Fix _filter_duplicates so pykg-config actually works.

$ PKG_CONFIG_PATH=/usr/lib/x86_64-linux-gnu/pkgconfig ./pykg-config --libs glib-2.0
Traceback (most recent call last):
  File "./pykg-config.py", line 377, in <module>
    main(sys.argv)
  File "./pykg-config.py", line 369, in main
    print result.get_all_lib_flags()
  File "/home/varg/pykg-config/pykg_config/result.py", line 181, in get_all_lib_flags
    return self._format_list(_filter_duplicates(result))
  File "/home/varg/pykg-config/pykg_config/result.py", line 77, in _filter_duplicates
    return [val for val in source if val not in locals()['_[1]']]
KeyError: '_[1]'

gbiggs/pykg-config,gbiggs/pykg-config