libubox: Lua binding needs mmu and fix coding style

Fixes
http://autobuild.buildroot.net/results/4e1/4e112e4463fef7ad8757259b222fab10fbd09d0d/



The Lua binding option of libubox uses fork() so it needs the MMU.

By the way, correction of coding style as stated in the buildroot
manual.

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>