Add config.log to gitignore globally

Add config.log to gitignore globally

When you end up explicitly regenerating chosen Makefiles by
running ../../config.status Makefile, the working tree ends up littered
with config.log files.

Currently, we only ignore config.log in the top directory - extend this
so that it's ignored across the tree.

Change-Id: If3cfec82fbf78e97ee769b36315ba23dcfe8a950
Reviewed-on: http://gerrit.openafs.org/2743
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>