selftests/damon: cleanup __pycache__/ with 'make clean'

selftests/damon: cleanup __pycache__/ with 'make clean'

Python-based tests creates __pycache__/ directory.  Remove it with 'make
clean' by defining it as EXTRA_CLEAN.

Link: https://lkml.kernel.org/r/20240827030336.7930-3-sj@kernel.org
Fixes: b5906f5f7359 ("selftests/damon: add a test for update_schemes_tried_regions sysfs command")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>