ANDROID: GKI: mm: Export symbols __next_zones_zonelist and zone_watermark_ok_safe

These symbols are required to modularize driver for
qcom,msm-ion.

Bug: 147914088
Change-Id: Ice7f76c6a52c6cc94ad7076a1c2396a90d6aa9ca
Signed-off-by: Hridya Valsaraju <hridya@google.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>