Add magit-section to pinned packages

This addresses the

  which-func-ff-hook error: (void-function magit-section--backward-find)

seen when `magit-status' is run in a buffer with imenu support on a
fresh Prelude checkout using the sample `prelude-pinned-packages.el'.

In magit 3.3.0, magit-section was spun off into its own package.
Later, magit commit 4727dcb removed the `magit-section--backward-find'
from magit-section.el.  Therefore, unless magit and magit-section are
in the same pinned or unpinned state the above error will occur.

sample/prelude-pinned-packages.el

@@ -83,6 +83,7 @@
         (macrostep . "melpa-stable")
         (magit . "melpa-stable")
         (magit-popup . "melpa-stable")
+        (magit-section . "melpa-stable")
         (makey . "melpa-stable")
         (markdown-mode . "melpa-stable")
         (marshal . "melpa-stable")