emacs: Autoload notmuch-jump-search only once

This function is being autoloaded using an autoload cookie, so it
shouldn't additionally be autoloaded using an `autoload' form.

When building libraries we don't actually load the autoloads file and
dropping the `autoload' form results in an error, which reveals a so
far unspecified dependency: `notmuch-tree' needs `notmuch-jump'.

Before this commit compiling (or even just loading) `notmuch-tree'
resulted in `notmuch-jump' being loaded because the former requires
`notmuch-lib', which autoloaded `notmuch-jump-search'.

The bug was that this dependency was not explicitly specified, which
we fix by adding the respective `require' form.