gnu: gnutls: Replace with 3.5.13.

This update addresses the following security advisories:

GNUTLS-SA-2017-3 (aka CVE-2017-7869) and GNUTLS-SA-2017-4.

See <https://gnutls.org/security.html> and <https://gnutls.org/news.html>.

* gnu/packages/patches/gnutls-skip-pkgconfig-test.patch,
gnu/packages/patches/gnutls-skip-trust-store-test.patch: New files.
* gnu/local.mk (dist_patch_DATA): Register patches.
* gnu/packages/tls.scm (gnutls)[replacement]: New field.
(gnutls-3.5.13): New variable.
(gnutls/guile-2.2)[replacement]: New field. Set #f.
[source]: Inherit from GNUTLS-3.5.13.