empty: needs MMU

The empty package uses the fork() system call, and so requires MMU.

Fixed autobuilder failures such as the following.

  http://autobuild.buildroot.net/results/4ad23e07b1bfe7bfd622aed8d54fa243b72eed59/build-end.log



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>