When building with DNS_DEBUG or ENABLE_ALL_THE_DEBUG_MACROS there is a
linking issue with Libraries/LibDNS/Resolver.h:1003 with
undefined reference DNS::Messages::Records::SIG::to_string() const
Add DNS_API on DNS:Messages::Records::SIG definition to allow access
to the class in bin/dns, bin/TestDNSResolver, libexec/RequestServer
This first pass only applies to the following two cases:
- Public functions returning a view type into an object they own
- Public ctors storing a view type
This catches a grand total of one (1) issue, which is fixed in
the previous commit.