Build system_ext build.prop with Soong

Bug: 322090587
Test: build and compare system_ext/etc/build.prop
Change-Id: I7b1af2e2b1fe7f107e1540d7b797c7bf489b8a0d
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 09ee938..5fca203 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -341,6 +341,7 @@
 $(call add_json_list, OemProperties, $(PRODUCT_OEM_PROPERTIES))
 
 $(call add_json_list, SystemPropFiles, $(TARGET_SYSTEM_PROP))
+$(call add_json_list, SystemExtPropFiles, $(TARGET_SYSTEM_EXT_PROP))
 
 # Do not set ArtTargetIncludeDebugBuild into any value if PRODUCT_ART_TARGET_INCLUDE_DEBUG_BUILD is not set,
 # to have the same behavior from runtime_libart.mk.