e1ae44b536
Having the definitions sit in the `net/gnrc/sixlowpan/frag.h` header does not make much sense, when using Selective Fragment Forwarding (and the fragmentation buffer already includes a `net/gnrc/sixlowpan/frag/stats.h` header), so they are moved to their own header. Since with this change it makes more sense to have the statistics stored in their own sub-module, the pseudo-module is also actualized.