package/gnutls: security bump to 3.6.13

Fixes the following security issue:

 * CVE-2020-11501: It was found that GnuTLS 3.6.3 introduced a
   regression in the DTLS protocol implementation. This caused the DTLS
   client to not contribute any randomness to the DTLS negotiation
   breaking the security guarantees of the DTLS protocol.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>