joe: needs mmu

Fixes:
http://autobuild.buildroot.net/results/f92/f920c85244afffe38497459753bf005946001d0d/

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

package/joe/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_JOE
 	bool "joe"
+	depends on BR2_USE_MMU # fork()
 	help
 	  Joe's Own Editor