drm/i915/oa: add content to Makefile

drm/i915/oa: add content to Makefile

Apparently the empty Makefile has caused some confusion. Add the
subdir-cc-flags-y as in 7fcc7ca549d4 ("drm/i915: add header search path
to subdir Makefiles") which should be useful.

The generated headers still aren't self-contained, so can't add that.

References: http://marc.info/?i=80bf2204-558a-6d3f-c493-bf17b891fc8a@infradead.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730113432.22146-1-jani.nikula@intel.com