build: Guard `system_file_diff_timestamp` with `USE_SOONG_DEFINED_SYSTEM_IMAGE`

* It's already guarded with `PRODUCT_SOONG_DEFINED_SYSTEM_IMAGE`,
  which is just... not enough.

Change-Id: I140faeee7d24917c234fc820a3ef7b53572b8cc9
diff --git a/core/Makefile b/core/Makefile
index 6d3fcf2..35c885d 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -3580,6 +3580,7 @@
 
 $(eval $(call write-partition-file-list,$(systemimage_intermediates)/file_list.txt,$(TARGET_OUT),$(FULL_SYSTEMIMAGE_DEPS)))
 
+ifeq ($(USE_SOONG_DEFINED_SYSTEM_IMAGE),true)
 ifneq ($(PRODUCT_SOONG_DEFINED_SYSTEM_IMAGE),)
 file_list_diff := $(HOST_OUT_EXECUTABLES)/file_list_diff$(HOST_EXECUTABLE_SUFFIX)
 system_file_diff_timestamp := $(systemimage_intermediates)/file_diff.timestamp
@@ -3606,6 +3607,7 @@
 
 $(BUILT_SYSTEMIMAGE): $(system_file_diff_timestamp)
 endif
+endif
 
 # Used by soong sandwich to request the staging dir be built
 $(systemimage_intermediates)/staging_dir.stamp: $(filter $(TARGET_OUT)/%,$(FULL_SYSTEMIMAGE_DEPS))