package/tar: Depends on MMU

Fixes
misc.o: In function `_xfork':
misc.c:(.text+0x614): undefined reference to `_fork'
../lib/libtar.a(rtapelib.o): In function `_rmt_open__':
rtapelib.c:(.text+0x6d2): 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>