Fix setup.py plugin_files

Fixes error in commit 25505c9216d84eee65ba1e230d6e9ce9600d5093
Revert unintended global change

made in commit 25505c9216d84eee65ba1e230d6e9ce9600d5093
Merge pull request #314 from Sophist-UK/sophist_fix_setup.py_windows_build

Fix setup.py plugin_files