Bump fhicl-cpp for art 1.17.x series
I am using FHiCL validation in LArSoft code, that still relies on
art 1.17 series (that is, ROOT 5).
When, and only if, a new
art 1.17 release is cut, I would like
fhiclcpp to be updated (to the current latest tag, I guess), to take advantage of newer features therein (in particular,
fhicl::OptionalAtome and company).
Note that I am not requesting a new release specifically for this issue.