when compiled with GSSAPI support, cache supported method OIDs by

when compiled with GSSAPI support, cache supported method OIDs by
calling ssh_gssapi_prepare_supported_oids() regardless of whether
GSSAPI authentication is enabled in the main config.

This avoids sandbox violations for configurations that enable GSSAPI
auth later, e.g.

Match user djm
	GSSAPIAuthentication yes

bz#2107; ok dtucker@