docs/core-api: Add Fedora instructions for GCC plugins

Add an example of how to install the necessary packages for GCC
plugins on Fedora.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>