Restore HTML_IMAGES to EXTRA_DIST

Commit 7d083dac removed HTML_IMAGES from
SETUP_FILES in order to handle the case where an image file is created
at build time in the builddir.  However, this caused HTML_IMAGES to be
removed from EXTRA_DIST as well, resulting in HTML_IMAGES not being
shipped in tarballs built with gtk-doc 1.17, and if built with
--enable-gtk-doc, the build of such packages fails.