Fix build issue when not using Kerberos.

Apparently when building without Kerberos support the entire contents of
camel-sasl-gssapi.c is omitted, which breaks generating API docs because
camel-scan.c expects a camel_sasl_gssapi_get_type() regardless.

With this commit, when building with Kerberos support, CamelSaslGssapi
is still a registered CamelSasl subclass but it does absolutely nothing
and is never used by Camel's SASL support.