ympd: Depends on MMU

This package needs MMU. Hence added dependency on BR2_USE_MMU

Fixes:
http://autobuild.buildroot.net/results/834/8340e69763e263fcf20e73f75eafcb31989e9f51//



Signed-off-by: Yuvaraj Patil <yuvaraj.patil@wipro.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>