improve Prolog vs. IDL detection (.pro files)

Apparently some .pro files are detected as Prolog, while others are given the IDL label.
The latter is related with the IDL/GDL/PV-WAVE language family.

In the GDL project, all .pro files are IDL source code, while they fall in both categories according to github:
- https://github.com/gnudatalanguage/gdl/search?l=prolog
- https://github.com/gnudatalanguage/gdl/search?l=IDL

Would be great to improve consistency (i.e., so that all are detected as IDL).

Likely relevant helper info:
https://github.com/blackducksoftware/ohcount/blob/master/src/parsers/idl_pvwave.rl

HTH,
Sylwester (originaly reported by @EdwardEisenhauer)