don't include "external-keyx" in case for setting username from GSSAPI

don't include "external-keyx" in case for setting username from GSSAPI
context later. if we don't set the username from the GSSAPI context
here for "external-keyx", we're not going to do it later either,
because the context should already be established from the key
exchange. only the "gssapi" userauth methods need to postpone setting
the username, as the GSSAPI context hasn't been established yet.