gnutls: make p11-kit and libtasn1 recommended

p11-kit is optional for gnutls to build, and only enables smartcard
support. Many Macs don't even have a smartcard slot. It definitely
isn't always required.

libtasn1 is a dependency of gnutls, but gnutls bundles a minimal
version of it with the source tarball, so it isn't strictly required,
as the current dependency setup would imply.