x11vnc: needs MMU

This package needs MMU. Hence this commit adds the dependency on
BR2_USE_MMU

Fixes:

  http://autobuild.buildroot.net/results/b9d/b9d27977a231d186fe5289232f5d3995ec756266/



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