private-etc: add gnutls dir to @network group

This is currently only present in `private-etc` in mutt.profile, though
it may also be used by other programs that use GNU TLS.

This was added to mutt.profile on commit a8a8e33 ("Add whitelisting to
mutt; improve geary, new profile for neomutt", 2020-12-28) / PR #3849.

Relates to #6400.

etc/profile-m-z/mutt.profile

@@ -127,7 +127,7 @@ tracelog
 #disable-mnt
 private-cache
 private-dev
-private-etc @tls-ca,@x11,Mutt,Muttrc,Muttrc.d,gai.conf,gnupg,gnutls,hosts.conf,mail,mailname,msmtprc,nntpserver,terminfo
+private-etc @tls-ca,@x11,Mutt,Muttrc,Muttrc.d,gai.conf,gnupg,hosts.conf,mail,mailname,msmtprc,nntpserver,terminfo
 private-tmp
 writable-run-user
 writable-var

src/include/etc_groups.h

@@ -79,6 +79,7 @@ static char *etc_group_sound[] = {
 static char *etc_group_tls_ca[] = {
 	"ca-certificates",
 	"crypto-policies",
+	"gnutls",
 	"pki",
 	"ssl",
 	NULL