Disable kdump build on linux 2.6

Disable kdump build on linux 2.6

Don't try to build kdump on linux 2.6 where it has been known to be broken
for a long time.  The build failure was ignored, but produced a large number
of errors and warnings, polluting the build log.
On a test amd64_linux26 system with a recent kernel and --enable-warnings,
this results in over 750 fewer warnings and 100+ fewer errors.

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/150
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>