linux: adjust fixup logic for ktap

The ktap package requires some parts of the kernel tracing
infrastructure to be enabled, especially
CONFIG_EVENT_TRACING. However, this option is a blind option in the
kernel, so enabling it in linux.mk has no effect: we need to enable a
non-blind option that selects CONFIG_EVENT_TRACING. We've chosen to
select CONFIG_ENABLE_DEFAULT_TRACERS.

This fixes the build of ktap.

[Thomas: use CONFIG_ENABLE_DEFAULT_TRACERS as suggested by Arnout.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>