adapt to DTXC_has_typedef_initialize_once change

dune/gdt/discretefunction/default.hh

dune/gdt/discretizations/interfaces.hh

dune/gdt/local/functionals/interfaces.hh

dune/gdt/operators/interfaces.hh

dune/gdt/spaces/cg/interface.hh

dune/gdt/spaces/dg/interface.hh

dune/gdt/spaces/fv/interface.hh

dune/gdt/spaces/interface.hh

dune/gdt/spaces/rt/interface.hh

dune/gdt/test/linearelliptic/problems/interface.hh

dune/gdt/type_traits.hh