fix typo in couchbaseS target

LCB_CORE_CFLAGS, not LCB_CORE_FLAGS

Change-Id: I1a616b7b481bd0c066917a6c3d86117186afa5bc
Reviewed-on: http://review.couchbase.org/42144
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>