selftests/zram: gitignore output file

After `make run_tests`, the git status complains:
Untracked files:
    (use "git add <file>..." to include in what will be committed)
        zram/err.log

This file will be cleaned up when execute 'make clean'

Link: https://lore.kernel.org/r/20241211004625.5308-1-lizhijian@fujitsu.com


Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>