package/gnutls: security bump to version 3.6.15

package/gnutls: security bump to version 3.6.15

libgnutls: Fixed "no_renegotiation" alert handling at incorrect timing.
The server sending a "no_renegotiation" alert in an unexpected timing,
followed by an invalid second handshake was able to cause a TLS 1.3
client to crash via a null-pointer dereference. The crash happens in the
application's error handling path, where the gnutls_deinit function is
called after detecting a handshake failure (#1071).
[GNUTLS-SA-2020-09-04, CVSS: medium]

https://lists.gnupg.org/pipermail/gnutls-help/2020-September/004669.html

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>