Revert "Remove Perl tidy backup files from .gitignore since malformed .pm.bak file break the compile test." – t/compile.t doesn't test *.bak files (http://search.cpan.org/~szabgab/Test-Strict-0.23/lib/Test/Strict.pm#all_perl_files_ok(_[_@directories_]_))

This reverts commit e96c4db67328e1c8cbac1f480136b784fd1f018e.