Clean and ignore additional build files for DocBook documentation

Clean and ignore additional build files for DocBook documentation

Depending on what generation method is used for the DocBook
documentation, additional build files may be created.  One of those
(*.aux) will cause the build to break if one switches from
docbook2pdf to dblatex without deleting it.  Update make clean
and .gitignore to remove and ignore these additional files.

Change-Id: I13cc8a3526bd7bc7c4c65cd7d2cd78b094398faa
Reviewed-on: http://gerrit.openafs.org/5089
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>