nut: needs mmu

While investigating on build failure [1], I realized that nut needs mmu since fork() is used.
So, conditions of this build failure are no longer encountered.

[1] http://autobuild.buildroot.net/results/961/96193e18782697c2af8813a7714b66b4ba1f3d1c//build-end.log



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>