Exclude the vtkgl.* files (generated code) from coverage

These files are generated, and pretty difficult to entirely cover
due to generating code for any and all OpenGL functions/extensions
that are possible after parsing the OpenGL headers.

Change-Id: I858056d3c6f3218edcb68f4ac93b7c41fa9d9385
Merge topic 'vtkgl-coverage' into master

8b534689 Exclude the vtkgl.* files (generated code) from coverage