php_couchbase.h should be minimal

It is included if you try to build it statically into the php
buildsystem and should therefore be minimal in order to
avoid name conflicts etc.

Change-Id: Ia321e3b6da5a04721b0b07f1320d6277f866615a
Reviewed-on: http://review.couchbase.org/25658
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>