dns/adns: Update to 1.6.0

Update dns/adns to 1.6.0.
This fixes several security issues in adns.
This fixes the build with -fno-common, which is the default with llvm 11.

PR: 248780
Approved by: pi@ (maintainer)
MFH: 2020Q3
Security: 08de38d2-e2d0-11ea-9538-0c9d925bbbc0