Add a misclassified Prolog file

This Prolog file was misclassified as Perl.  I assume linguist
was confused because the file has many comments.  Nevertheless,
there are plenty of Prolog-distinguishing tokens such as `:-`,
`module`, `%%`, capitalized variables names, `foo/2`, etc.
Merge pull request #321 from mndrix/patch-1

Add a misclassified Prolog file