overlay: Update .gitignore

Fixes: 865a47ca ("overlay: parse tracepoints from sysfs to figure out fields' location")
Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>

overlay/.gitignore

 intel-gpu-overlay
+tracepoint_format.h
 kms/.dirstamp
 x11/.dirstamp