Enforce /system module RRO for mainline products

Turn on RRO enforcement for /system modules for products
that use mainline_system.mk

Bug: b/137727426
Test: compare_images
Change-Id: Ia1824481c85fb031d5e156307bf7a848e4721d9e
diff --git a/target/product/mainline_system.mk b/target/product/mainline_system.mk
index 43bc45f..721f5d9 100644
--- a/target/product/mainline_system.mk
+++ b/target/product/mainline_system.mk
@@ -108,6 +108,8 @@
 PRODUCT_PACKAGES += \
     com.android.apex.cts.shim.v1_prebuilt
 
+PRODUCT_ENFORCE_RRO_TARGETS := *
+
 PRODUCT_NAME := mainline_system
 PRODUCT_BRAND := generic