Force PRODUCT_ENABLE_UFFD_GC to true on GSI.

If the kernel version isn't available to the build system and
PRODUCT_ENABLE_UFFD_GC isn't explicitly set, there will be a build
warning. To address the warning and make the behavior more explicit on
GSI builds, we need this change.

Bug: 321751629
Test: lunch aosp_x86_64-trunk_staging-userdebug && m dist
Change-Id: I5b73890235827929ea3bce5a07260d6c67a00a27