ltris: needs MMU

This package needs MMU. Hence added dependency on BR2_USE_MMU

Fixes:
http://autobuild.buildroot.net/results/5cf/5cf2262460a72ce51b479a42fca61cea5f6e23fd//



Signed-off-by: Yuvaraj Patil <yuvaraj.patil@wipro.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>