XML-RPC: Check if category support was added for pages before calling wp_get_post_categories() to avoid unnecessary query in wp.getPage.

props markoheijnen, nprasath002.
fixes #17920.
Built from https://develop.svn.wordpress.org/trunk@29731


git-svn-id: http://core.svn.wordpress.org/trunk@29505 1a063a9b-81f0-0310-95a4-ce76da25c4cd