ANDROID: mm: Fix implicit declaration of function 'isolate_lru_page'

When compiled with CONFIG_SHMEM=n, shmem.c does not include internal.h
and isolate_lru_page function declaration can't be found.
Fix this by making isolate_lru_page usage conditional upon CONFIG_SHMEM
inside reclaim_shmem_address_space.

Fixes: daeabfe7 ("ANDROID: mm: add reclaim_shmem_address_space() for faster reclaims")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Ia46a57681d26ac103e84ef7caa61a22dbd45cf04