list-c++, set-c++, oset-c++, map-c++, omap-c++: Don't fool the compiler. Reported by Akim Demaille in <https://lists.gnu.org/archive/html/bug-bison/2020-05/msg00102.html>. * lib/gl_list.hh (gl_List::iterator::next): Avoid a reinterpret_cast. * lib/gl_set.hh (gl_Set::iterator::next): Likewise. * lib/gl_oset.hh (gl_OSet::iterator::next): Likewise. * lib/gl_map.hh (gl_Map::iterator::next): Likewise. * lib/gl_omap.hh (gl_OMap::iterator::next): Likewise.