selftests: core: add unshare_test to gitignore

This executable is missing from the corresponding gitignore file.
Add unshare_test to the core gitignore list.
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/core/.gitignore

 close_range_test
+unshare_test