Silence compiler warnings new with gcc 8.

* dirmngr/dns.c: Include gpgrt.h.  Silence -Warray-bounds also gcc.
* tests/gpgscm/scheme.c: Include gpgrt.h.
(Eval_Cycle): Ignore -Wimplicit-fallthrough.

The funny use of case and labels in the CASE macro seems confuse the
fallthrough detection.

• Signed-off-by: Werner Koch <wk@gnupg.org>