mdadm: needs mmu

Fixes:
http://autobuild.buildroot.net/results/52c/52cc56df2aa278d92141918de74833d7e4fd3d47/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mdadm/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_MDADM
 	bool "mdadm"
+	depends on BR2_USE_MMU # fork()
 	help
 	  Utility for managing Linux Software RAID arrays.