coreutils: needs mmu

Fixes:
http://autobuild.buildroot.net/results/bb975d928c08d31d0c1ecfb9519034865120b244/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/coreutils/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_COREUTILS
 	bool "coreutils"
 	depends on BR2_USE_WCHAR
+	depends on BR2_USE_MMU # fork()
 	help
 	  All of the basic file/text/shell utilities.  These are the
 	  core utilities which are expected to exist on every system.