libubox: Disable lua binding on no mmu

The Lua binding option of libubox need to be
explicitly disabled on no mmu case.

Fixes:
http://autobuild.buildroot.net/results/1a5/1a5c50f8ca0ae70c1e0d3733765f521790b97a30/build-end.log



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>