A pointer is not 32 bit on all platforms.
Since gnrc_lwmac only stores 16 bit in the pointer variable it is
still save to cast like this even on AVR, but cast to uintptr_t
instead of uint32_t.
fixes#12869
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`)