sys/gnrc/sixlowpan/frag/fb: guard sfr_types.h header include

[fjmolinas]

Contribution description

This guards the header.

Testing procedure

• green murdock
• in master the following error showed up if using ztimer_xtimer_compat (no 64bit), which exposed the wrongful inclusion.

USEMODULE=ztimer_xtimer_compat make -C tests/gnrc_sixlowpan_frag -j

In file included from /home/francisco/workspace/RIOT/sys/include/evtimer_msg.h:24,
                 from /home/francisco/workspace/RIOT/sys/include/net/gnrc/sixlowpan/frag/sfr_types.h:25,
                 from /home/francisco/workspace/RIOT/sys/include/net/gnrc/sixlowpan/frag/fb.h:31,
                 from /home/francisco/workspace/RIOT/sys/net/gnrc/network_layer/sixlowpan/frag/fb/gnrc_sixlowpan_frag_fb.c:20:
/home/francisco/workspace/RIOT/sys/include/evtimer.h: In function ‘evtimer_now_msec’:
/home/francisco/workspace/RIOT/sys/include/evtimer.h:134:12: error: implicit declaration of function ‘xtimer_now_usec64’; did you mean ‘xtimer_now_usec’? [-Werror=implicit-function-declaration]
  134 |     return xtimer_now_usec64() / US_PER_MS;
      |            ^~~~~~~~~~~~~~~~~
      |            xtimer_now_usec

Note: the above command still yields an error, but will be fixed in another PR.

Issues/PRs references

Found in #17365

[kaspar030]

ACK.