imagemagick: needs mmu

Fixes:
http://autobuild.buildroot.net/results/d2f/d2f673b71f771ab5a6435805e39338fd023f04c8/build-end.log



The complete log file says:
libMagickCore-6.Q16.so: undefined reference to `_fork'

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>