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

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/[email protected]
Cc: Shuah Khan <[email protected]>
Signed-off-by: Li Zhijian <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

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