xorg-server: needs mmu

os/utils.c::System() unconditionally uses fork(), so it only builds on
configurations with mmu.

(Indirectly) fixes:
http://autobuild.buildroot.net/results/497/4970352598a68d7af89acbb33cdf69d656e23f4d/
http://autobuild.buildroot.net/results/d27/d279dc35bff53b7f939af02bee70b1ed3014e49b/
http://autobuild.buildroot.net/results/84b/84b8f19221a5c841de3016959c99b92c96c57e81/



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