Fix windows build: use "lcb_assert.h" header

iocp_loop.obj : error LNK2019: unresolved external symbol lcb_assert referenced in function iocp_run
iocp_util.obj : error LNK2001: unresolved external symbol lcb_assert
libcouchbase.dll : fatal error LNK1120: 1 unresolved externals

Change-Id: I10e6f868f4925a73abe3f1f49b64c32b8b003922
Reviewed-on: http://review.couchbase.org/28201
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>