ANDROID: fix kernelci build failure in vmscan.c

Vendor hooks added in vmscan.c directly referenced a vendor-specific
field which is only defined if CONFIG_ANDROID_VENDOR_OEM_DATA is
enabled. A kernelci config wich CONFIG_ANDROID_VENDOR_OEM_DATA
disabled and CONFIG_ANDROID_VENDOR_HOOKS enabled has a build-break
due to the undefined field.

Fixes: 3e2dc32f ("ANDROID: mm: create vendor hooks for memory reclaim")
Change-Id: Id7d31af9cf5752eba5ba27c5d31a288230f29114
Signed-off-by: Todd Kjos <tkjos@google.com>