Add $(TARGET_DEVICE_DIR)/system.prop to be a prerequisite

If $(TARGET_DEVICE_DIR)/system.prop exists, it should be
a prerequisite of $(INSTALLED_BUILD_PROP_TARGET),
as the following rules state.

Change-Id: Ie395b6f08cef198c3f5c733a5b6ae5a458335a9c
diff --git a/core/Makefile b/core/Makefile
index 852a716..142a2c1 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -112,7 +112,7 @@
 endef
 
 BUILDINFO_SH := build/tools/buildinfo.sh
-$(INSTALLED_BUILD_PROP_TARGET): $(BUILDINFO_SH) $(INTERNAL_BUILD_ID_MAKEFILE)
+$(INSTALLED_BUILD_PROP_TARGET): $(BUILDINFO_SH) $(INTERNAL_BUILD_ID_MAKEFILE) $(wildcard $(TARGET_DEVICE_DIR)/system.prop)
 	@echo Target buildinfo: $@
 	@mkdir -p $(dir $@)
 	$(hide) TARGET_BUILD_TYPE="$(TARGET_BUILD_VARIANT)" \