security: compile capabilities by default

Capabilities have long been the default when CONFIG_SECURITY=n,
and its help text suggests turning it on when CONFIG_SECURITY=y.
But it is set to default n.

Default it to y instead.

Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Matt LaPlante <kernel1@cyberdogtech.com>
Signed-off-by: James Morris <jmorris@namei.org>