package/exim: uses fork(), needs an MMU

Fixes:
    http://autobuild.buildroot.net/results/95d/95ddd8e9be96f3808612db02642d72cfe77b3c26/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Romain <Naour&lt;romain.naour@openwide.fr>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>