gnutls: allow gnutls_aead_cipher_encryptv2 with gcm before 3.6.15

gnutls: allow gnutls_aead_cipher_encryptv2 with gcm before 3.6.15

The memory leak bug up to 3.6.14 was only related to ccm, but gcm was
fine.

This avoids talloc+memcpy on more systems, e.g. ubuntu 20.04,
and brings ~ 20% less cpu overhead, see:
https://hackmd.io/@asn/samba_crypto_benchmarks

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14764

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 5512416a8fbe00a7a5343afe0d50846e0a8f342b)