Fix error: unused parameter ‘itr’

[saleyn]

exprtk.hpp:2418:58: error: unused parameter ‘itr’ [-Werror=unused-parameter]
 2418 |          inline bool is_comment_start(details::char_cptr itr)
      |                                       ~~~~~~~~~~~~~~~~~~~^~~

[ArashPartow]

Hello @saleyn

Thank-you for reporting this issue. I hadn't performed a combinatorial set of builds based around the various defines for some time, as a result this issue had gotten through.

That being said, I decided to resolve the issue a little differently, essentially making it a little more readable:

22beb73#diff-58c1983215ef11e39375e044aaa946c840e57c7f72d3ba9e303ee1a9b7cd9865R2418