devel/jwt-cpp: Fix emplacement of cmake files

jwt-cpp installed .cmake files in ${PREFIX}/cmake which
is not really the standard place where it should be.
This patch fix the emplacement where it should be to
be consistant with other ports.
Added devel/jsoncpp as build dependency.

Approved by: 0mp (mentor)
Differential Revision: https://reviews.freebsd.org/D50578