add .gitreview to .gitignore

Ignore .gitreview configuration file. It must be placed
in the cloned repo directory in order for git-review
to use it when submitting / fetching changes.

Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Change-Id: I5f027c3208bafcfd16ac980b42a314a2914f5036
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4547


Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>