make.dep/gcoap: remove gnrc dependency

[kb2ma]

Contribution description

This PR is a simple followup to #12931 to remove gcoap's dependency on the gnrc_sock_udp module. gcoap already depends on sock_util, which depends on sock_udp. #12931 uses the sock_udp dependency to bring in gnrc_sock_udp only when the gnrc module is used.

This PR is the first in a series to allow use of the lwIP stack with gcoap, as you can see in my gcoap/lwip_for_example branch.

Testing procedure

Compile the gcoap example, and compile and run the gcoap unit tests.

Issues/PRs references

Based on #12931.

[miri64]

Mh, but as a long as the stack doesn't use mbox it won't compile this way, right?

[kb2ma]

I had to pick a sequence for the commits in gcoap/lwip_for_example branch. Would you rather see ee1eede and e800be5 first?

[miri64]

I'm sorry, but I have to honestly say: Neither! Preferably, I'd like to see gcoap not using sock-struct internal members, as it is intended. That's why I provided sock_async with the last release :-).

[miri64]

With the current approach you just moving the problem further to lwIP. Any new stack implementing sock would now need to now add an extra adaption layer for gcoap.

[kb2ma]

I understand your perspective on integration of sock_async into gcoap. On the other hand, I needed to get gcoap working on IPv4 ASAP, and the solution here works today. I plan to maintain this work myself then, but added a note to #8130 for anyone else interested.

[kb2ma]

Closing in favor of #13386, but still need sock_async for lwIP.