xmlsec-gnutls: reading keys from memory in PEM format not implemented

[jas4711]

With the GnuTLS backend, OATH-Toolkit fails to load keys and fails like this:

func=xmlSecGCryptAppKeyLoadMemory:file=app.c:line=236:obj=unknown:subj=unknown:error=9:feature is not implemented:details=xmlSecKeyDataFormatPem func=xmlSecGCryptAppKeyLoad:file=app.c:line=189:obj=unknown:subj=xmlSecGCryptAppKeyLoadMemory:error=1:xmlsec library function failed:filename=./pskc-ee-key.pem