add PRODUCT_SOURCE_ROOT_DIRS product variable

Change-Id: I31590eb133511a5dbdb8d6fe436cf511d1a160a2
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 0101796..0c6ab70 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -306,6 +306,7 @@
 $(call add_json_bool, IgnorePrefer32OnDevice, $(filter true,$(IGNORE_PREFER32_ON_DEVICE)))
 
 $(call add_json_list, IncludeTags,                $(PRODUCT_INCLUDE_TAGS))
+$(call add_json_list, SourceRootDirs,             $(PRODUCT_SOURCE_ROOT_DIRS))
 
 $(call json_end)