gnu: seahorse: Update to 3.30.1.1.

* gnu/packages/gnome.scm (seahorse)[version]: Update version number.
[origin]: Update hash.  Add patch needed for compilation with our
version of libsecret (0.19.1).
[inputs]: Add avahi dependency.
[native-inputs]: Add vala dependency.  Use gettext-minimal instead of
intltool, as only xgettext is used.
* gnu/packages/patches/seahorse-gkr-use-0-on-empty-flags.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>