selftests/damon: fix damon selftests by installing _common.sh

_common.sh was recently introduced but is not installed and then triggers
an error when trying to run the damon selftests:

selftests: damon: sysfs.sh
./sysfs.sh: line 4: _common.sh: No such file or directory

Install this file to avoid this error.

Link: https://lkml.kernel.org/r/20250812-alex-fixes_manual-v1-1-c4e99b1f80e4@rivosinc.com


Fixes: 51191450 ("selftests/damon: introduce _common.sh to host shared function")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Tested-by: Enze Li <lienze@kylinos.cn>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>