LU-2989 build: some make targets are broken

LU-2989 build: some make targets are broken

Add make targets "distclean" and "maintainer-clean" in
ldiskfs/ldiskfs/Makefile. The global "make distclean" now
executes whithout an error.

Also lustre/target/.gitignore file is created to hide
message from GIT.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: Ic8841d11960d5aacd8293ecfcaf8c628dd743c45
Reviewed-on: http://review.whamcloud.com/5766
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alexey Shvetsov <alexxy@gentoo.org>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>