xapp_rstart: Depends on MMU

Fixes
  CCLD   rstartd.real
auth.o: In function `_do_auth':
auth.c:(.text+0x6a): undefined reference to `_fork'
server.o: In function `_detach':
server.c:(.text+0x80a): undefined reference to `_fork'

To reproduce use this defconfig
http://autobuild.buildroot.net/results/8d5/8d5ad404fab7ec2a501972ba9c08b0109e420b80/defconfig



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>