mm/Kconfig: Re-add HAVE_USERSPACE_LOW_MEMORY_KILLER

When Android has a userspace low memory killer, we want to configure
certain parts of the reclaim paths in the kernel.
HAVE_USERSPACE_LOW_MEMORY_KILLER can be used to perform this configuration,
and was removed when reverting a patch that defined it.  Re-add it.

Change-Id: Ic069c92a9d7928f0402705df8375d5384e9f743c
Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>