libvncserver: needs mmu

Fixes http://autobuild.buildroot.net/results/30e/30e6a879ab6d84df4688c3d29761522f68dad9fc/



fork() is only used in VNCommand.c, so it seems doable to patch the sources
(and send upstream) to make it work on !mmu if people are interested.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>