gnu: libmicrohttpd: Use 'gnutls/dane' as input.

This switches the used GnuTLS to the one recommended
by the GNUnet Documentation (Chapter "Installation Handbook",
Section "Generic installation instructions") where a specific
order of dependency installation is required.  In this
order libunbound is installed first, then GnuTLS, then libgnurl,
followed by libmicrohttpd.

* gnu/packages/gnunet.scm (libmicrohttpd)[inputs]: Replace gnutls with
  'gnutls/dane'.

Signed-off-by: Christopher Baines <mail@cbaines.net>