emacs: Work around gnus-inhibit-images bug in mm-shr

Emacs 24's mm-shr HTML email renderer fails to load gnus-art before
referencing gnus-inhibit-images, resulting in a void-variable error
when notmuch attempts to render an HTML email with inline images.
This works around this bug by advising mm-shr to load gnus-art.

mm-shr is the only function outside of gnus-art itself that references
gnus-inhibit-images, so this workaround should be correct.  If this
ever changes, hopefully they will have fixed this bug upstream first.

This fixes the "Rendering HTML mail with images" test for Emacs 24.