LU-10997 build: add files to .gitignore

LU-10997 build: add files to .gitignore

To avoid by accident adding files created during the build
process add them to the .gitignore. For Ubuntu 18 we add
in .cache.mk and *.o.ur-safe which only show up on that
platform

Test-Parameters: trivial

Change-Id: Ie1329e765f080cbdac1bd3efdd63f83a65d45989
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/32304
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>