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>

package/imagemagick/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_IMAGEMAGICK
 	bool "imagemagick"
+	depends on BR2_USE_MMU # fork()
 	help
 	  ImageMagick(R) is a software suite to create, edit, and compose
 	  bitmap images. It can read, convert and write images in a variety of