devel/thrift-c_glib: Disable -Werror and fix build

src/thrift/generate/t_py_generator.cc:1603:38: error: unused variable 'a_iter' [-Werror,-Wunused-variable]

vector<t_field*>::const_iterator a_iter;
                                 ^

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p501065_s347375/logs/errors/thrift-c_glib-0.11.0,1.log

PR: 230607