build: Add gitattribute file to build-aux

build: Add gitattribute file to build-aux

The command: `make check-tabs` fails on Windows due to line ending conversions
caused by the following setting: `git config --global core.autocrlf true`
(the whitelist `build-aux/initial-tab-whitelist` becomes a blacklist)

This patch adds a .gittatribute file to build-aux to force LF endings
on Windows.

Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>
Co-authored-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Aaron Conole <aconole@redhat.com>
Acked-by: Ben Pfaff <blp@ovn.org>