php.extensions.couchbase: Fix build and runtime loading

The couchbase extension depends on the igbinary PECL which needs to be
loaded and loaded prior to it. It also seems like the pcs extension
isn't actually needed - it at least builds and loads without it. Since
the pcs extension dependency was the reason couchbase didn't build on
PHP 7.4 it now does, so let's unmark it broken.