math/rubygem-rb-gsl: Add port.

Ruby/GSL is a Ruby extension library for GSL (GNU Scientific Library),
which provides a number of functions and methods for numerical
computing in Ruby.

WWW: https://rubygems.org/gems/rb-gsl

PR: 196994
Submitted by: Shin-ya MURAKAMI <murashin@gfd-dennou.org>
Approved by: mentor (implicit)