... and fix compile errors.

Replace the usage of `showSDocUnsafe` with `showSDoc dflags`
in example code in extending_ghc.rts.

This example contains several compile errors (missing import
and syntax error), this patch also fixes that.

Test Plan: [skip ci]

Reviewers: bgamari, mpickering

Reviewed By: mpickering

Subscribers: mpickering, rwbarton, thomie, carter

GHC Trac Issues: #15228

Differential Revision: https://phabricator.haskell.org/D4815