Correctly forward-declare inline function headers in generic-node.h

R=bmeurer@chromium.org, jarin@chromium.org

Review URL: https://codereview.chromium.org/488363003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23331 ce2b1a6d-e550-0410-aec6-3dcde31c8c00