Whilelist system/product_services in gsi_common.mk

GSI could include system/product_services to override the content
in product_services partition, so whitelist the folder.

Bug: 119747870
Test: build a build target has prodcut_services and
Test: inherit gsi_common.mk
Change-Id: Ied201aad88d75ef85258c24e8a2a4952b968ff3f