ANDROID: fix kernelci build-break in lowmemorykiller

<linux/poll.h> was not included by lowmemorykiller.c
resulting in a kernelci build-break due to no definition
for 'poll_table'.

Fixes: 941128efa146c ("ANDROID: Communicates LMK events to
userland where they can be logged")
Signed-off-by: Todd Kjos <tkjos@google.com>

Change-Id: I5fb2d1d8f6e5afbdd00fbf017df37ec2e8b9ff9e