Preprocesor fails to evaluate the if condicion on L91-92 because
RTT_FREQUENCY is not defined, and therefore a division by 0 occurs.
TO avoid this replicate the RTT_FREQUENCY undefined warning.
The inclusion of `net/gnrc.h` in `net/gnrc/mac/types.h` header makes it
impossible to include the `net/gnrc/netif.h` header within
`net/gnrc/netif/hdr.h`, due to `net/gnrc/mac/types.h` being included
with `net/gnrc/netif/mac.h` (which is included in `net/gnrc/netif.h`)
