src: do not include internals from node_buffer.h

`node_buffer.h` is a public header, so it should not be using
the `node_internals.h` internal header.

Ref: 290315ac
Fixes: https://github.com/nodejs/node/issues/15552
PR-URL: https://github.com/nodejs/node/pull/15554


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>