pkg/tinydtls: Only include contrib folder if using tinydtls_sock_dtls

[leandrolanzieri]

Contribution description

As described in #13121, the contrib folder in tinydtls package is always included, which needs definitions for sock (sock_types.h).

This PR modifies the Makefile to only add this directory to the build if the module tinydtls_sock_dtls is used, which is the only case when the folder is needed.

Testing procedure

• examples/dtls-echo and examples/dtls-sock should still work as usual
• The issue presented in tinydtls package has undeclared dependencies #13121 should be fixed

Issues/PRs references

Fixes #13121

[chrysn]

This solves my problem, both in the minimal case of #13121 and the practical code of libOSCORE. The DTLS echo example still works as well. I did not test this in combination with #12104, but as they pull in tinydtls_sock_dtls as well, I don't see why it shouldn't work.

Looks like just the right solution. Thank you!

[cgundogan]

Tested together with @leandrolanzieri

USEPKG=tinydtls make for gnrc_networking should fail without this PR and it compiles with this PR.