fi_trigger: direct include for trigger context

The trigger context can be overriden by the provider in
with --enable-direct, similar to fi_context.

Install fi_direct_trigger.h file.

Signed-off-by: Sayantan Sur <sayantan.sur@intel.com>
Merge pull request #292 from sayantansur/fi-direct-trigger

fi_trigger: direct include for trigger context