sconeserver: missing dependency on BR2_USE_MMU

The BR2_PACKAGE_SCONESERVER_HTTP_SCONESITE_IMAGE option of sconeserver
selects imagemagick, but forgets to depends on BR2_USE_MMU, which
imagemagick depends on.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>