gnrc_ipv6: Multicast is not forwarded if routing node listens to the address

[DipSwitch]

Currently if you send a broadcast to ff05::1 for example a routing node doesn't forward the packed if the router itself has the address assigned to it's receiving interface.

I'll create a pull-request later today to beter illustrate what I mean. :) However, thoughts on the subject are welcome :)

[DipSwitch]

Closed in favor of #4528

[miri64]

Normally we connect an issue and a pull request by commenting "Fixes #". This way GitHub closes the issue as soon as it is fixed (i.e. the PR was merged.)

[miri64]

#4528 was adapted in #5729, but it seems not seem to fix this issue...