platform.h needed for ssize_t on WIN32
platform.h needed for ssize_t on WIN32

ssize_t doesn't exist on WIN32, we need to include
<platform/platform.h> to get an alternative definition. Add the
appropriate header here, so couchstore header files are standalone
(given that kv_engine is cleaning up its headers to remove unnecessary
includes, which was previously causing file_ops.h to get platform.h
"for free").

Change-Id: I554125c82604d02150ab062c28f478a0caaf9eef
Reviewed-on: http://review.couchbase.org/80373
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

couchbaselabs/couchstore,couchbaselabs/couchstore,couchbaselabs/couchstore