Since #include <gsettingsbackend.h> is a perfectly valid thing for
applications to do, and since we want to include gio headers from
gsettingsbackend.h, we need to effectively disable the #error we would
get from those headers (because we're not coming via gio.h).

We don't want to #include <gio/gio.h> here because this would cause
needless rebuilding of GSettingsBackend, GSettings,
GDelayedSettingsBackend, etc... every time someone changed anything in
any public header.