package/matchbox: Depends on MMU

Fixes matchbox-panel:
http://autobuild.buildroot.net/results/8d5/8d5ad404fab7ec2a501972ba9c08b0109e420b80/



and matchbox-desktop:
mbdesktop_win_plugin.o: In function `mbdesktop_win_plugin_load':
/home/br/br/output/build/matchbox-desktop-0.9.1/src/mbdesktop_win_plugin.c:22: undefined reference to `_fork'

and matchbox-wm:
matchbox-remote.o: In function `mbcommand':
/home/fli4l/br3/output/build/matchbox-wm-1.2/src/matchbox-remote.c:147: undefined reference to `_fork'

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>