Use += to append to target-specific PRIVATE_CLEAN_FILES

Instead of incorrectly including global variable PRIVATE_CLEAN_FILES

Change-Id: I9b5e12448dad5001de051a566d8a94a89b20ecac
diff --git a/core/dynamic_binary.mk b/core/dynamic_binary.mk
index 0ca1858..6ede57f 100644
--- a/core/dynamic_binary.mk
+++ b/core/dynamic_binary.mk
@@ -128,8 +128,7 @@
 endif # LOCAL_STRIP_MODULE
 
 
-$(cleantarget): PRIVATE_CLEAN_FILES := \
-			$(PRIVATE_CLEAN_FILES) \
-			$(linked_module) \
-			$(symbolic_output) \
-			$(compress_output)
+$(cleantarget): PRIVATE_CLEAN_FILES += \
+    $(linked_module) \
+    $(symbolic_output) \
+    $(compress_output)