core: don't hardcode "out" during findleaves.py invocations.

"out" can be $OUT_DIR

Bug: 7183057
Change-Id: I7b52d40de4398f481e47f6d6d719fb550d874f73
diff --git a/core/cleanspec.mk b/core/cleanspec.mk
index d4a8eed..500ce54 100644
--- a/core/cleanspec.mk
+++ b/core/cleanspec.mk
@@ -64,6 +64,6 @@
 # ************************************************
 
 subdir_cleanspecs := \
-    $(shell build/tools/findleaves.py --prune=out --prune=.repo --prune=.git . CleanSpec.mk)
+    $(shell build/tools/findleaves.py --prune=$(OUT_DIR) --prune=.repo --prune=.git . CleanSpec.mk)
 include $(subdir_cleanspecs)
 subdir_cleanspecs :=